litedown: R Markdown Reimagined

Yihui Xie

2024-11-15

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Preface

The litedown package is still new and experimental. The documentation is very incomplete. Besides, litedown was designed for minimalists with a limited scope. Average users should perhaps consider using rmarkdown or Quarto instead.

You may say I’m a dreamer
But I’m not the only one
I hope someday you’ll join us
And the world will live as one

—John Lennon, Imagine

Imagine there’s no PDF. It’s easy if you try. No Word below us. Above us, only HTML.

I do not mean PDF and Word are bad. I only lament the time that human beings spend on various document formats, given how much a static web page can do.

The past journey

Having worked on the development of R Markdown for nearly 12 years, I have to confess that I have become a little perplexed about my work, although I clearly remember the exciting moments. For example:

I think the work was largely meaningful. The only problem is I do not see an end. The list of exciting things to do goes on and on.

A question

“At forty, I had no more doubts.” Confucius said. Apparently, I’m not Confucius. On the contrary, as I’m turning forty, I’m having more doubts. I have been reflecting on the things that have kept me busy.

If I were to summarize the 700+ episodes of Naruto in one sentence, I would use the question that Gaara (the Fifth Kazekage) asked Onoki (the Third Tsuchikage):

When did you all forsake yourselves?

In my opinion, this question was the most critical turning point in the 700+ episodes, reminding the 79-year-old Onoki and the rest of people of their original dreams. The shinobi world existed to put an end to wars, but it turned out to bring even more and larger-scale wars.

When did we forsake simplicity? We create software to simplify things, but software often ends up in feature wars.

Markdown was originally invented for simplicity. That is, to make it easier to write HTML. I used to take out my wallet and tell people that if they are unable to learn the basics of Markdown in 5 minutes, I’d award them 20 dollars. I also used to say “In HTML I Trust”, which sounds like a joke, but I really love HTML and web technologies. Again, I do not mean other document formats are bad at all. It is just that I feel HTML has the greatest potential, and I hope to take full advantage of its power.1

In other words, I do not expect that “the (software) world will live as one”. I just want to make the HTML world a little bit better.

Overview

The litedown package is an attempt to reimagine R Markdown with one primary goal—do HTML, and do it well, with the minimalism principle. Before LaTeX fans walk away in disappointment, let me quickly clarify that LaTeX output is also supported, but please do not expect anything fancy before you learn to customize LaTeX templates. Most other output formats are not supported. No Word, RTF, PowerPoint, or EPUB.

R Markdown is rendered via litedown::fuse(), which is similar to rmarkdown::render() and knitr::knit(). Markdown is rendered via litedown::mark(), which uses commonmark instead of Pandoc.

The commonmark package follows the GFM (GitHub Flavored Markdown) spec, which can be seen as a subset of Pandoc’s Markdown. Therefore the litedown package can be viewed as a small subset of the current R Markdown ecosystem (the latter is based on Pandoc). It aims at simplicity, lightweight, and speed, at the cost of giving up some advanced features. This package is intended for minimalists. Most users may prefer using tools based on Pandoc instead, such as rmarkdown or Quarto, which offer richer features.

Is litedown really simple? From the developer’s perspective, yes, it is, largely due to the limited scope of the package. From the user’s perspective, some features are definitely not that simple. However, the point is that the core is simple and small, and you can enable or disable most features. What’s more, you can implement features by yourself if you know CSS/JS.

Highlights

Small footprint

Almost everything in litedown was written from scratch. The package is very lightweight. Currently, it has two dependencies, commonmark and xfun. It does not depend on knitr or Pandoc.

It is a deliberate design choice to keep this package lightweight, to make it relatively easy to use and simple to maintain. The functions mark() and fuse() can be viewed as significantly trimmed-down versions of Pandoc and knitr, respectively.

It is absolutely not the goal for litedown to become a substitute of tools based on knitr and Pandoc, such as rmarkdown and Quarto. If you are not sure if you should choose litedown or rmarkdown/Quarto, you may want to choose the latter (especially Quarto). Some litedown features may be ported into knitr in the future.

Live preview everything

To get started, run litedown::roam() and it will launch a file browser to let you preview everything that can be rendered to HTML, such as .md, .Rmd, and .R files.

The rendering takes place in memory, which means it will not render .html files on your disk, unless you request so. The page will be automatically updated as you edit and save a file. This update-on-save feature can be turned off, and then you can manually refresh the page to re-render the file whenever you want.

You can open any file in your editor or the default system application by clicking a button in the browser. You can also render a file or a project in a new R session by clicking a button in the browser.

Precise parser

The R Markdown parser stores the precise line and column numbers of code elements. The location information is used in various places. For example, when an error occurs, you will get a message that tells you the precise location in the source. In editors that support ANSI links (such as the RStudio IDE), you can even click on the message to go to a specific line/column in the source document, so you can quickly and easily know where the error occurred exactly. When previewing .Rmd documents with roam(), you will see line numbers on the left side of code blocks. Clicking on these numbers will bring you to the lines in the source.

Due to the fact that the parser is based on commonmark (instead of solely on regular expressions like knitr’s parser), it can precisely recognize code chunks and inline code, which means code within other verbatim code blocks or comments will be untouched. For example:

````md
Below is not a code chunk but verbatim content
inside a fenced code block (with four backticks).

```{r}
1 + 1
```

Inline code expressions like `{r} 1+1` are not
parsed, either.
````

<!--
Feel free to comment out anything, such as a code chunk:

```{r}
1 + 1
```
-->

Versioned CSS/JS assets

By default, litedown produces bare minimal HTML output, but many features can be enabled by adding CSS/JS assets (see 4). You can freely choose whatever versions that work for you to avoid potential future breakage when the assets are upgraded. The assets are not bundled in the litedown package, but hosted on CDN, so updating litedown will not update your assets.

The CSS and JS code for commonly used features do not depend on any frameworks such as Bootstrap or jQuery. They are simply vanilla code written from scratch. No generator was used (such as SCSS). The code is often relatively short, so you could just fork the code, modify it, publish your own version, and use it if you are not satisfied with the original version.

Chunk options management

Chunk options are managed by an environment, i.e., litedown::reactor(). Using an environment (as compared to a list like knitr’s opts_chunk) means you can access the up-to-date chunk options anywhere, because environments in R are mutable. I cannot explain how awkward knitr’s opts_current has been. It is basically a lie—chunk options that you get from opts_current are not necessarily “current”.

Non-linear order of execution

A document does not have to be compiled in the linear order. With the chunk option order, you can specify code chunks and inline code expressions to be executed in an arbitrary order. For example, if you have an abstract in the beginning of a document, and the abstract needs to use a number calculated at the end of the document, you can let the abstract be compiled in the end, although it appears earlier in the document.

I guess some users may want to kill me upon knowing this feature, and some may send me flowers (although I’m not sure if they want to thank me or prepare the flowers for my funeral). For those who want to kill me, please note that this feature does not mean litedown is as awful as Jupyter notebooks. It means you can specify a fixed order to execute code in the document. The order does not have to be from beginning to end, but it is deterministic. In other words, it does not mean you run code chunks in an arbitrary or random order that can detriment reproducibility.

Time code chunks

If you want to figure out which code chunks are time-consuming, simply set the chunk option litedown::reactor(time = TRUE) in the beginning, and put litedown::timing_data() in the last code chunk. It will tell you detailed information. In the roam() preview, the data will also contain links to specific lines of code chunks, so you click to jump to a specific code chunk.

Table output for data frames by default

Rectangular objects such as data frames (including tibbles) and matrices are printed as tables by default, and the number of rows is limited to 10 by default to avoid generating huge tables by accident.

Relieved pain of paths

File paths (such as image paths) have been a mess in knitr. My deepest apologies for that. I have worked much harder in litedown in this regard.

Output in memory or to disk

Functions such as mark(), fuse(), and fuse_book() can operate in memory without writing to disk. By default, if you pass a file input, you get a file output; if you pass text input, you get text output.

A new cache system

You can feel more confident with using the chunk option cache = TRUE in litedown than in knitr. The new cache system is more robust and intelligent.

A book is a single HTML file

Unlike bookdown and Quarto, litedown::fuse_book() renders multiple input files to a single output file. Yes, your whole precious book is in a single (HTML or PDF) file. Grab and go.

The assumption of single-file output for books has made several things a lot easier. For example, if you want to search within the book, just press Ctrl + F or Command + F in your browser as you usually do on any other web page. You do not need a client-side search library. It is also quicker to jump between chapters because they are on the same page. If you want to print the HTML version of the book to PDF, just press Ctrl + P or Command + P.

I know you have a concern: wouldn’t the single HTML file be too heavy for the browser? The answer is: you should be fine if you do not have too many images. If you do, do not base64 encode them (which is the default), and you can also lazy-load images to make the book load faster.

R scripts as first-class citizens

An R script has the same status as an R Markdown document. You can write Markdown in #' comments, and chunk options in #| comments (both are optional). These are the only two rules to remember. Compared to knitr::spin(), the rules are much simpler. Besides, R scripts are also carefully parsed into “code chunks”, so their line numbers work as well as R Markdown’s line numbers.

Any application that you can create with R Markdown can be created with R scripts, such as books and websites.

Clean HTML output

The HTML output generated from litedown is very clean. For example, code blocks in HTML output contain plain code, instead of full of <span> tags with random attributes. Clean HTML output means the output file size is smaller, and more importantly, it is easier to inspect the differences between two versions of output (e.g., in Git). Every time when you update the source document, you can know more clearly what has changed in the output, which can help you avoid unexpected changes before publishing the output.

Features out of scope

Output formats besides HTML and LaTeX are unlikely to be supported.2 If other output formats are desired, you may use Pandoc to do the conversion.

For tables, only pipe tables are supported. Other table formats are not recognized.

At the moment, litedown mainly supports R as the computing language. Other languages might be added in the future, but please keep your expectation low, because the support is unlikely to be as good as R.

HTML widgets are not supported yet, but may be reimagined in the future with some minimal support.

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1 Knitting, or Fusing

R Markdown documents need to be knitted to Markdown before being rendered to a target output format. The function litedown::fuse() plays a role similar to knitr::knit(). It “fuses” program code with narratives, i.e., it executes all code in the source document and interweaves results with narratives in the output document. Similar to knitr, litedown supports code chunks and inline code.

1.1 Code chunks

A code chunk consists of the language name, chunk options (in the chunk header or pipe comments), and the code:

```{lang}
#| chunk options

code
```

Currently, a subset of knitr chunk options are supported, which can be accessed in litedown::reactor(). To get an option value, use reactor("NAME"), where NAME is the option name (e.g., fig.width). To set an option globally, use reactor(NAME = VALUE). Alternatively, you can manipulate the value returned by reactor() directly, which is essentially an environment:

opts = litedown::reactor()
opts$fig.width  # get an option
opts$echo = FALSE  # set an option

Code chunks inside other code blocks are not parsed or evaluated, which provides a way to write verbatim code chunks, e.g.,

````markdown
Some verbatim content.

```{r}
1 + 1
```
````

Similarly, code in comments will not be recognized, either, e.g.,

<!--
Do not run this chunk:

```{r}
1 + 1
```

or the inline code `{r} pi`.
-->

If N + 1 pairs of curly braces are used in the opening fence, the chunk fences (with N pairs of curly braces) and chunk options will be shown in the output, which can be useful for telling readers the full source of a chunk, e.g.,

```{{r}}
#| echo = TRUE, eval = FALSE

1 + 1
```

1.2 Inline code

1.2.1 The syntax

The syntax for inline code expressions is `{lang} code`, where lang is the language name, e.g., r. Spaces are allowed after the opening backtick and before the closing backtick. If the code happens to contain N backticks, you will need to use at least N + 1 backticks outside, e.g., ``{r} paste("`", rnorm(1), "`")``. An inline code expression inside another piece of inline code is not parsed or evaluated, which provides a way to write verbatim inline code expressions, e.g., `` `{lang} code` ``.

Comma-separated chunk options can also be provided to inline code expressions after the language name, e.g., `{r, eval=FALSE} code`.

1.2.2 Compatibility with knitr

For knitr users, please note that the syntax `r code` is not supported by default. You have to wrap the language name r in curly braces. As a temporary workaround, you may set options(litedown.enable.knitr_inline = TRUE) in your .Rprofile to make litedown recognize `r code`, but we recommend that you convert the document via litedown:::convert_knitr() instead if you decide to stay with litedown in the long run.

1.2.3 Numeric output

If the inline expression returns a single number, the number will be formatted. To bypass the formatting, wrap the inline expression in I(). We denote the number by \(x\) for now.

1.3 R scripts

Besides R Markdown, you can also pass an R script to fuse(). You can write Markdown content in #' comments, and group lines of code into chunks by #| comments, e.g.,

#' ---
#' title: An R script
#' output:
#'   litedown::latex_format: null
#' ---
#' 
#' A _paragraph_.

#| eval=FALSE
1:10
1 + 1

#| fig.width=10, dev='pdf'
plot(cars)

Both #' and #| comments are optional.

1.4 Language engines

Currently, the main computing language supported by litedown is R. You can check all supported languages via:

names(litedown::engines())

#> [1] "css"     "mermaid" "js"      "md"      "r"

You can get a language engine definition via litedown::engines('LANG'), where LANG is the language name, e.g.,

litedown::engines('md')

You can also set a new language engine via:

litedown::engines(LANG = function(x, inline = FALSE, ...) {

})

The function’s argument x is an element in the list returned by litedown::crack().

1.4.1 The Markdown engine

The md engine will output Markdown text both verbatim and as-is, which can be useful for showing Markdown examples, e.g.,

```{md}
You can see both the _source_ and _output_ of
this `md` chunk.
```

You can also use `{md} the engine **inline**`.

1.4.2 The CSS/JS engines

You can insert CSS/JS to the output via the css/js engines, e.g.,

```{css}
a {
  color: red;
}
```

```{js}
document.body.classList.add('dark');
```

1.4.3 The Mermaid engine

The mermaid engine can be used to draw diagrams with mermaid.js, e.g.,

```{mermaid, flow-abcd, echo=FALSE, fig.cap='A flowchart.'}
graph TD;
    A-->B;
    A-->C;
    B-->D;
    C-->D;
```

The JS library mermaid.js has to be loaded for the diagrams to be rendered. By default, the latest version of mermaid.js will be added to the js variable (3.3.2.2) in the YAML metadata automatically. If you prefer using a specific version, you can add it manually to the YAML metadata, e.g.,

---
title: "A mermaid diagram"
output:
  litedown::html_format:
    meta:
      js: ["@npm/[email protected]/dist/mermaid.min.js"]
---

```{mermaid}

```

1.5 Comparison to knitr

Major differences between knitr and litedown include:

knitr litedown
Supports multiple graphical devices for a chunk. Only supports one device for a chunk (but there are multiple choices for this device).
Depending on certain chunk options, figure output could be both Markdown (![]()) and raw HTML (<img>) / LaTeX (\includegraphics{}). Always use Markdown syntax for figures.
The document parser is based on regular expressions and not robust. Code chunks and inline expressions are not aware of their contexts (e.g., code blocks or comments). The parser is based on commonmark, which is more robust and makes it straightforward to write verbatim code (in a parent code block) or comment out code (in <!-- --> comments).
Supports a large number of chunk options and language engines. Supports a limited number of chunk options and engines.
Inline code does not support options or languages other than R. Inline code supports options and other languages.
All code is executed in the linear order. Code chunks and inline code can be executed in a custom non-linear order defined by the chunk option order (higher values indicate higher priority).
Supports chunk hooks and output hooks. No hooks at the moment.
The package is more than 12 years old and quite mature. The package is new and still experimental.

If you feel any indispensable features are missing in litedown, please feel free to suggest them in Github issues. However, please remember that the goal of litedown is not to fully re-implement rmarkdown, knitr or Pandoc. Some features may never be re-implemented, especially when the implementation is not simple enough.

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2 Markdown Syntax

2.1 Basic syntax

For the full list of supported document elements, please read the GFM spec. Below is a quick summary:

2.2 Add-on features

In addition to GFM features, the litedown package also supports the following features.

2.2.1 Raw LaTeX/HTML blocks

Raw LaTeX and HTML blocks can be written as fenced code blocks with language names =latex (or =tex) and =html, e.g.,

```{=tex}
This only appears in \LaTeX{} output.
```

Raw LaTeX blocks will only appear in LaTeX output, and will be ignored in other output formats. Similarly, raw HTML blocks will only appear in HTML output. One exception is raw LaTeX blocks that are LaTeX math environments, which also work for HTML output (see the next section).

2.2.2 LaTeX math

You can write both $inline$ and $$display$$ LaTeX math, e.g., \(\sin^{2}(\theta)+\cos^{2}(\theta) = 1\).

$$\bar{X} = \frac{1}{n} \sum_{i=1}^n X_i$$

$$|x| = \begin{cases} x &\text{if } x \geq 0 \\ -x &\text{if } x < 0 \end{cases}$$

For expressions in pairs of single or double dollar signs to be recognized as LaTeX math, there must be no spaces after the opening dollar sign, or before the closing dollar sign. The math expression should either start from the very beginning of a line, or have a space or ( before the opening dollar sign.

Valid examples:

$x + y$
($x + y$)
  $x + y$
text $x + y$ text

$$x + y$$
  $$x + y$$
text $$x + y$$ text
$$x +
  y$$

Invalid examples:

$ x + y$  <- space after the opening `$`
text$x + y$  <- lack of space before the opening `$`
text $x + y$10 text  <- number after closing `$`
$x +
y$  <- multi-line `$ $` expressions
$`x + y`$  <- expression wrapped in backticks

$$x +
  y
$$
^- lack of non-space character before closing `$$`

LaTeX math environments are also supported, e.g., below are an align environment and an equation environment:

\begin{align}
a^{2}+b^{2} & =  c^{2}\\
\sin^{2}(\theta)+\cos^{2}(\theta) & =  1
\label{eq:pyth-identity}
\end{align}

\begin{equation}
  \begin{split}
  (a+b)^2 &=(a+b)(a+b)\\
    &=a^2+2ab+b^2
  \end{split}
\end{equation}

\begin{align} a^{2}+b^{2} & = c^{2}\\ \sin^{2}(\theta)+\cos^{2}(\theta) & = 1 \label{eq:pyth-identity} \end{align}

\begin{equation} \begin{split} (a+b)^2 &=(a+b)(a+b)\\ &=a^2+2ab+b^2 \end{split} \end{equation}

These math environments can be written as either nake LaTeX code or raw LaTeX blocks (```{=latex}), but we recommend that you use raw LaTeX blocks because they are more robust. LaTeX math environments work for both LaTeX and HTML output.

For HTML output, it is up to the JavaScript library (MathJax or KaTeX) whether a math environment can be rendered (3.1.6).

2.2.3 Superscripts and subscripts

Write superscripts in ^text^ and subscripts in ~text~ (same syntax as Pandoc’s Markdown), e.g., 210 and H2O. Currently only alphanumeric characters, *, (, and ) are allowed in the scripts. For example, a^b c^ will not be recognized as a superscript (because the space is not allowed). Note that GFM supports striking out text via ~text~, but this feature has been disabled and replaced by the feature of subscripts in litedown. To strike out text, you must use a pair of double tildes.

2.2.4 Footnotes

Insert footnotes via [^n], where n is a footnote number (a unique identifier). The footnote content should be defined in a separate block starting with [^n]:. For example:

Insert a footnote here.[^1]

[^1]: This is the footnote.

The support is limited for LaTeX output at the moment,4 and there are two caveats if the document is intended to be converted to LaTeX:

The two limitations do not apply to HTML output, e.g., you can write arbitrary elements in footnotes and not necessarily one paragraph.

2.2.5 Attributes

Attributes on images, links, fenced code blocks, and section headings can be written in {}. ID and class are two common attributes. An ID can be written after the # character, and a class can be written after . . Attributes are typically written in the form name="value", and separated by spaces (in fact, you can also write IDs and classes explicitly like other attributes, e.g., id="foo" class="bar"). Certain attributes do not require values, and you can provide the attribute name only, e.g., disabled or contenteditable, although it is harmless to write disabled="true".

Only lowercase letters (a-z), digits (0-9), hyphens (-), and colons (:) are allowed in ID and class strings. For example, sec:intro and fig-cars are valid IDs, but sec_intro and tab cars are not.

For example, ![text](path){.foo #bar width="50%"} will generate an <img> tag with attributes in HTML output:

<img src="path" alt="text" id="bar" class="foo" width="50%" />

and ## Heading {#baz} will generate:

<h2 id="baz">Heading</h2>

Links of the form [text](url){...} will generate:

<a href="url" ...></a>

When the url is empty, <a> will be converted to <span>, e.g., [text](){.foo .bar} will generate:

<span class="foo bar">text</span>

This provides a way to create <span> elements, which is similar to bracketed Spans (i.e., [text]{...}) in Pandoc’s Markdown.

For fenced code blocks, a special rule is that the first class name will be treated as the language name for a block, and the class attribute of the result <code> tag will have a language- prefix. For example, the following code block

```{.foo .bar #my-code style="color: red;"}
```

will generate the HTML output below:

<pre>
  <code class="language-foo bar" id="my-code" style="color: red;">
  </code>
</pre>

Most attributes in {} are ignored for LaTeX output except for:

2.2.6 Appendices

When a top-level heading has the attribute .appendix, the rest of the document will be treated as the appendix. If section numbering is enabled (3.1.9), the appendix section headings will be numbered differently.

2.2.7 Fenced Divs

A fenced Div can be written in ::: fences. Note that the opening fence must have at least one attribute, such as the class name. For example:

::: foo
This is a fenced Div.
:::

::: {.foo}
The syntax `::: foo` is equivalent to `::: {.foo}`.
:::

::: {.foo #bar style="color: red;"}
This div has more attributes.

It will be red in HTML output.
:::

A fenced Div will be converted to <div> with attributes in HTML output, e.g.,

<div class="foo" id="bar" style="color: red;">
</div>

For LaTeX output, it can be converted to a LaTeX environment if both the class name and an attribute data-latex are present. For example,

::: {.tiny data-latex=""}
This is _tiny_ text.
:::

will be converted to:

\begin{tiny}
This is \emph{tiny} text.
\end{tiny}

The data-latex attribute can be used to specify arguments to the environment (which can be an empty string if the environment doesn’t need an argument). For example,

::: {.minipage data-latex="{.5\linewidth}"}

will be converted to:

\begin{minipage}{.5\linewidth}

The data-latex attribute is optional for fenced Divs with class names figure or table. They will be converted to figure or table environments. For example,

:::: {.figure}
![](foo.png)

::: {.fig-caption}
This is a caption.
:::
::::

will be converted to:

\begin{figure}
  \includegraphics{foo.png}
  \caption{This is a caption.}
\end{figure}

Other fenced Divs will be ignored if they don’t have the data-latex attribute, and their inner content will be written out normally without a surrounding environment.

If a fenced Div has multiple class names (e.g., {.a .b .c}), only the first class name will be used as the LaTeX environment name. However, all class names will be used if the output format is HTML (e.g., <div class="a b c">).

2.2.8 Cross-references

To cross-reference an element, it must be numbered first. Then we can refer to it by its ID.

2.2.8.1 Sections, figures, and tables

Section heading IDs can be either manually assigned or automatically generated (3.1.1). Section numbers are automatically generated if the number_sections option is true (3.1.9).

Figures and tables are automatically numbered if their captions are provided (via the chunk options fig.cap / tab.cap), e.g.,

```{r}
#| nice-plot, fig.cap="A nice caption"

plot(cars)
```

To refer to an element in the text, use the syntax @ID, where ID is the ID of the element to be referenced, which typically consists of a prefix (e.g., sec:, fig:, tab:, or eq:) and a label. For example:

Please see @fig:nice-plot for an overview of the `cars` data.

Hyphens (-) are also allowed in place of colons in the ID prefix, e.g., @fig-nice-plot.

2.2.8.2 LaTeX equations

LaTeX math environments such as align and equation are numbered by default. To refer to an expression (e.g., an equation) in a math environment, a label needs to be assigned to the expression first via \label{}, and it must start with the prefix eq: or eq-, e.g.,

```{=latex}
\begin{equation}
\sin^2(x) + \cos^2(x) = 1 \label{eq:pyth-identity}
\end{equation}
```

Then we can use either @eq:* or @eqn:* to cross-reference the equation, e.g., @eqn:pyth-identity. Under the hood, the prefix @eq will be resolved to \ref{}, and @eqn will be resolved to \eqref{}. If you are not familiar with LaTeX commands \ref{} and \eqref{}, the main difference is that \eqref{} will render the equation number in parentheses, e.g., \(\eqref{eq:pyth-identity}\), whereas \ref{} will only generate the equation number, e.g., \(\ref{eq:pyth-identity}\).

In HTML output, \eqref{} will also add a label “Equation” before the number by default. If you prefer writing the label manually and having control over the parentheses, you can use @eq: instead of @eqn:, e.g., Eq. [@eq:*] (using the label “Eq.” and square brackets).

In addition to equation numbers, you can also specify a tag for an equation via \tag{} and refer to the equations by its tag, e.g.,

For a right-angled triangle, we are all familiar with @eqn:pyth-theorem.

\begin{equation}
a^2 + b^2 = c^2 \label{eq:pyth-theorem} \tag{PT}
\end{equation}

For a right-angled triangle, we are all familiar with \(\eqref{eq:pyth-theorem}\).

\begin{equation} a^2 + b^2 = c^2 \label{eq:pyth-theorem} \tag{PT} \end{equation}

2.2.8.3 Arbitrary elements

You can cross-reference any other type of elements by adding empty anchors of the form [](#@ID) into them, e.g.,

::: {style="background: ghostwhite; padding: 1px 1em;"}
[](#@blk:example) This is a numbered block.
:::

We can cross-reference @blk:example.

1 This is a numbered block.

We can cross-reference 1.

For HTML output, we can style the numbers and references with CSS. Element numbers are wrapped in <span class="ref-number-*"></span>, and references are wrapped in <span class="cross-ref-*"></span>, where * is the ID prefix (e.g., fig, tab, and eqn). For example, we can add the label “Block” to the number and reference of the block above:

.ref-number-blk::before, .cross-ref-blk::before {
  content: "Block ";
}
.ref-number-blk {
  font-weight: bold;
  font-style: italic;
}
.ref-number-blk::after {
  content: ". "
}

2.2.9 Citations

This feature requires the R package rbibutils. Please make sure it is installed before using citations.

xfun::pkg_load2("rbibutils")

To insert citations, you have to first declare one or multiple bibliography databases in the YAML metadata, e.g.,

bibliography: ["papers.bib", "books.bib"]

Each .bib file contains entries that start with keywords. For example, R-base is the keyword for the following entry:

@Manual{R-base,
  title = {R: A Language and Environment for Statistical Computing},
  author = {{R Core Team}},
  organization = {R Foundation for Statistical Computing},
  address = {Vienna, Austria},
  year = {2024},
  url = {https://www.R-project.org/},
}

Then you can use [@R-base] or @R-base to cite this item. The keywords must consist of only alphanumeric characters (a-z, A-Z, 0-9) and -. You can include multiple keywords in [ ] separated by semicolons.

For HTML output, the citation uses the author-year style. The syntax [@keyword] generates the citation in parentheses (e.g., (Author, 2024)), and @keyword only puts the year in parentheses (e.g., Author (2024)).

For LaTeX output, the citation style depends on the LaTeX package, which you can set via the citation_package option of the output format in YAML metadata, e.g.,

output:
  litedown::latex_format:
    citation_package: biblatex

The default is natbib. The table below shows the LaTeX commands corresponding to the Markdown citation syntax:

citation package [@key-1; @key-2] @key
none \cite{key-1, key-2} \cite{key}
natbib \citep{key-1, key-2} \citet{key}
biblatex \parencite{key-1, key-2} \cite{key}

Note that litedown actually generates \citep and \citet regardless of the citation package, but will redefine them to \cite or \parencite according to the citation package. For example, it will insert \let\citep\parencite in the LaTeX preamble when citation_package is biblatex.

2.2.10 Smart HTML entities

“Smart” HTML entities can be represented by ASCII characters, e.g., you can write fractions in the form n/m. Below are some example entities:

1/2 1/3 2/3 7/8 1/7 1/9 1/10 (c) (r) (tm)
½ ⅓ ⅔ ⅞ ⅐ ⅑ ⅒ © ® ™

2.3 Comparison to Pandoc

As mentioned earlier, a lot of features in Pandoc’s Markdown are not supported in the litedown package. Any feature that you find missing in previous sections is likely to be unavailable. In addition, a lot of R Markdown and Quarto (both are based on Pandoc) features are not supported, either. Some HTML features have been implemented via JavaScript and CSS.

Pandoc can convert Markdown to many output formats, such as Word, PowerPoint, LaTeX beamer, and EPUB. The litedown package is unlikely to support output formats beyond HTML and LaTeX.

Edit this chapter on GitHub

3 Markdown Rendering

The main function to convert Markdown to other formats is litedown::mark().

You can either call litedown::mark() to render a Markdown document programmatically, or click the Knit button in RStudio to render a (Markdown or R Markdown) document interactively. The latter requires you to specify the output format in the output field in YAML metadata (3.3), e.g.,

---
output:
  litedown::html_format:
    options:
      js_math:
        package: "katex"
        version: "0.16.4"
      number_sections: true
      embed_resources: ["local", "https"]
    meta:
      css: "custom.css"
---

3.1 Markdown options

The options argument of mark() can be used to enable/disable/set options to control Markdown rendering. This argument can take either a list, e.g., list(toc = TRUE, smart = FALSE), or a character vector, e.g., c("+toc", "-smart"), or equivalently, +toc-smart, where + means to enable an option, and - means to disable an option. The options can also be set in YAML metadata in 3.3 (recommended). Available options are listed below.

3.1.1 auto_identifiers

Add automatic IDs to headings, e.g.,

# Hello world!

## Introduction

will be converted to

<h1 id="chp:hello-world">Hello world!</h1>

<h2 id="sec:introduction">Introduction</h2>

The prefix chp: (chapter) will be added to the automatic IDs of level-one headings, and sec: (section) will be added for other levels of headings.

You can override the automatic ID by providing an ID manually via the ID attribute, e.g.,

# Hello world! {#hello}

An automatic ID is generated by substituting non-alphanumeric characters in the heading text with hyphens. If the result is empty, the ID will be section. If any ID is duplicated, a numeric suffix will be added to the ID, e.g., example_1 and example_2.

3.1.2 cleveref

Whether to use the LaTeX package cleveref for “clever” cross-references (2.2.8). This option is for LaTeX output only. If enabled, cleveref will be loaded and references will use the command \cref{} instead of \ref{}, which will automatically add the type of reference before the reference number, e.g., \cref{sec:intro} may generate Section 1, so you do not have to write Section \ref{sec:intro}.

3.1.3 embed_cleanup

Whether to clean up plot files after they have been embedded in HTML output (see 3.1.4).

3.1.4 embed_resources

Embed resources (images, CSS, and JS) in the HTML output using their base64-encoded data (images) or raw content (CSS/JS). Possible values are:

The default is "local", i.e., local resources are embedded, whereas https resources are not. This means the output document may not work offline. If you have to view the output offline, you need to use the option value "https" (or "all") and render the document at least once before you go offline.

3.1.5 js_highlight

Specify the JavaScript library to syntax highlight code blocks. Possible values are highlight (highlight.js) and prism (Prism.js). The default is prism. This option can also take a list of the form list(package, version, style, languages), which specifies the package name (highlight or prism), version, CSS style/theme name, and names of languages to be highlighted.

By default, languages are automatically detected and the required JS files are automatically loaded. Normally you need to specify the languages array only if the automatic detection fails.

Technically this option is a shorthand for setting the metadata variables css and js in 3.3. If you want full control, you may disable this option (set it to false or null) and use metadata variables directly, which requires more familiarity with the JS libraries and the jsdelivr CDN.

3.1.6 js_math

Specify the JavaScript library for rendering math expressions in HTML output. Possible values are "mathjax" and "katex" (the default). Like the js_highlight option, this option is also essentially a shorthand for setting the metadata variables css and js.

If you want finer control, you can provide a list of the form list(package, version, css, js). This will allow you to specify the package name, version, and css/js files. For example, if you want to use MathJax’s tex-chtml.js instead, you may set:

js_math:
  package: mathjax
  version: 3
  js: es5/tex-chtml.js

By default, MathJax version 3 is used. If you want to use the older v2, you may set:

js_math:
  package: mathjax
  version: 2
  js: MathJax.js?config=TeX-AMS-MML_CHTML

Please visit the MathJax CDN to know which versions and JS files are available.

For KaTeX, the version is not specified by default, which means the latest version from the CDN. Below is an example of specifying the version 0.16.4 and using the mhchem extension:

js_math:
  package: katex
  version: 0.16.4
  js: [dist/katex.min.js, dist/contrib/mhchem.min.js]

Note that if you want the HTML output to be self-contained via the embed_resources option, KaTeX can be embedded and used offline, but MathJax cannot be fully embedded due to its complexity. MathJax v3 can be partially embedded and used offline, but currently only its fonts can be embedded, and extensions cannot. If you must view HTML output offline, we recommend using KaTeX, but please also note that KaTeX and MathJax do not fully cover each other’s features.

3.1.7 keep_yaml

Whether to keep the YAML metadata in the output. When TRUE, the original YAML in the Markdown input (if exists) will be written to the output. Note that when this option is enabled, templates (3.2) will be disabled and mark() will only generate a document fragment. This option was introduced mainly for Hugo websites to use litedown instead of Hugo’s Markdown engines to render pages.

3.1.8 latex_math

Whether to identify LaTeX math expressions in pairs of single ($ $) or double dollar signs ($$ $$), and transform them so that they could be correctly rendered by MathJax (HTML output) or LaTeX.

3.1.9 number_sections

Whether to number section headings. To skip numbering a specific heading, add a class attribute .unnumbered (or use the shorthand -) to it. For example:

## Preface {.unnumbered}

## About the author {-}

3.1.10 smartypants

Whether to translate certain ASCII strings into smart typographic characters (see ?litedown::smartypants).

3.1.11 superscript

Whether to translate strings between two carets into superscripts, e.g., text^foo^ to text<sup>foo</sup>.

3.1.12 subscript

Whether to translate strings between two tildes into subscripts, e.g., text~foo~ to text<sub>foo</sub>.

3.1.13 toc

Whether to generate a table of contents (TOC) from section headings. If a heading has an id attribute, the corresponding TOC item will be a link to this heading. You can also set a sub-option:

To exclude a certain heading from the TOC, assign a class name unlisted to it. For example:

## Acknowledgments {.unlisted}

3.1.14 top_level

The desired type of the top-level headings in LaTeX output. Possible values are 'chapter' and 'part'. For example, if top_level = 'chapter', # heading will be rendered to \chapter{heading} instead of the default \section{heading}.

Options not described above can be found on the help pages of commonmark, e.g., the hardbreaks option is for the hardbreaks argument of commonmark::markdown_*() functions, and the table option is for the table extension in commonmark’s extensions.

litedown::markdown_options()
#>  [1] "-cleveref"         "-hardbreaks"       "-number_sections" 
#>  [4] "-smartypants"      "-tagfilter"        "-toc"             
#>  [7] "+auto_identifiers" "+autolink"         "+cross_refs"      
#> [10] "+embed_cleanup"    "+embed_resources"  "+js_highlight"    
#> [13] "+js_math"          "+latex_math"       "+smart"           
#> [16] "+strikethrough"    "+subscript"        "+superscript"     
#> [19] "+table"            "+tasklist"        
# commonmark's arguments
opts = formals(commonmark::markdown_html)
opts = opts[setdiff(names(opts), c('text', 'extensions'))]
unlist(opts)
#> hardbreaks      smart  normalize  sourcepos  footnotes 
#>      FALSE      FALSE      FALSE      FALSE      FALSE 
# commonmark's extensions
commonmark::list_extensions()
#> [1] "table"         "strikethrough" "autolink"      "tagfilter"    
#> [5] "tasklist"

3.2 Templates

By default, mark() generates a document fragment (i.e., the body) if the input does not contain YAML metadata at the beginning. To generate a full document, you need to specify YAML metadata. A full document is generated with a template. Below is a simple HTML template example:

<html>
  <head>
    <title>$title$</title>
  </head>

  <body>
  $body$
  </body>
</html>

It contains two variables, $title$ and $body$. All variables will be substituted by metadata values, except for $body$, which is from the body of the input document (after conversion to a target output format).

The litedown has provided default templates for HTML and LaTeX output. To pass metadata to templates, use the meta argument, e.g.,

litedown::mark(..., meta = list(title = "My Title"))

If you want to use a custom template file, you can set the path in the global option litedown.FORMAT.template (where FORMAT is the output format name (html or latex), e.g., in .Rprofile:

options(litedown.html.template = 'path/to/my/template.html')

The global option will be applied to all documents to be converted by mark(). Alternatively, you can pass a template path to the template argument of the output format litedown::html_format or litedown::latex_format in an individual document, e.g.,

---
output:
  litedown::html_format:
    template: "path/to/my/template.html"
---

The template path can also take a logical value: TRUE means to use the default template, and FALSE means to generate only a fragment document without using any template.

3.3 YAML metadata

Alternatively, the meta argument can read YAML metadata in the Markdown document.

3.3.1 Top-level variables

The following variables can be set in the top-level fields in YAML:

For example:

---
title: "My Title"
author: "[Frida Gomam](https://example.com)"
date: "2023-01-09"
---

The values are treated as Markdown text, and will be converted to the target output format before being passed to the template. For the above example, the variable $author$ will be <a href="https://example.com">Frida Gomam</a> in the HTML template.

For top-level variables, mark() will also create the “underscore” versions for templates, which contain HTML and LaTeX markups. For example, for the $title$ variable, $title_$ will also be available to the template. The following table shows the values of the underscore variables, assuming the original value of a variable is TEXT:

Variables HTML LaTeX
$title_$ <div class="title"> <h1>TEXT</h1> </div> \title{TEXT}
$subtitle_$ <div class="subtitle"> <h2>TEXT</h2> </div> \subtitle{TEXT}
$author_$ <div class="author"> <h2>TEXT</h2> </div> \author{TEXT}
$date_$ <div class="date"> <h3>TEXT</h3> </div> \date{TEXT}
$abstract_$ <div class="abstract"> <p>TEXT</p> </div> \begin{abstract} TEXT \end{abstract}

If an original variable is empty or missing, its underscore version will also be empty. For the $author_$ variable, if the $author$ variable contains multiple author names as an array, each name will be in a separate <h2> in HTML output, and all names will be concatenated by \and in LaTeX output, e.g., for

author: ["Jane X", "John Y"]

in YAML, the HTML output will be:

<div class="author">
  <h2>Jane X</h2>
  <h2>John Y</h2>
</div>

and the LaTeX output will be:

\author{Jane X \and John Y}

If you design your own template, you are free to use either the original or the underscore versions of these variables. For example, you could put the title in an <h1> without the <div> wrapper via <h1 class="title">$title$</h1> instead of using $title_$.

When these top-level variables are also provided as meta variables for an output format, the latter will override the former, e.g.,

title: "Global Title"
output:
  litedown::html_format:
    meta:
      title: "Title for HTML output"
  litedown::latex_format:
    meta:
      title: "Title for LaTeX output"

3.3.2 Format-specific variables

Other variables need to be specified under output -> litedown::*_format -> meta, where * can be html or latex, e.g.,

---
title: "My Title"
output:
  litedown::html_format:
    meta:
      css: "style.css"
      js: "script.js"
  litedown::latex_format:
    meta:
      documentclass: "book"
      header_includes: "\\usepackage{microtype}"
---

The following metadata variables are supported for both HTML and LaTeX templates:

The following variables are for HTML templates:

3.3.2.1 The css variable

A vector of CSS files to be included in the output. The default value is litedown:::pkg_file('resources', 'default.css').

If you want to use built-in CSS files in this package, you can only specify the base name, e.g., default means default.css in this package.

You can also use web resources, e.g., https://example.org/style.css. One special case is jsdelivr resources: if a css value starts with @, it will be recognized as a jsdelivr.com resource. if you are not familiar with jsdelivr, you may read its documentation to understand the following example URLs. The shorthand syntax is as follows (CDN stands for https://cdn.jsdelivr.net):

This provides a way to reduce the output HTML file size by loading CSS from the web instead of embedding inside HTML, at the cost of requiring Internet connection when viewing the HTML file. If you need the external web resources to work after you go offline, you can enable "https" in the Markdown option embed_resources in advance to embed the resources.

3.3.2.2 The js variable

A vector of JavaScript files to be included in the output. The syntax is the same as the css variable, e.g., snap means snap.js in this package, @snap means a “jsdelivr” resource, and you can use arbitrary paths or URLs to other JS files.

3.3.2.3 The body-class variable

A class name for the main body (the default value is body).

The following variables are for LaTeX templates:

3.3.2.4 The classoption variable

A string containing options for the document class.

3.3.2.5 The documentclass variable

The document class (by default, article).

3.3.3 Naming convention of variables

A variable name must consist of alphanumeric characters and hyphens only. You could use underscores in variable names in the metadata, but please note that underscores will be normalized to hyphens internally, e.g., header_includes will be converted to header-includes. This means if you use a custom template, you must use hyphens instead of underscores as separators in variable names in the template.

3.3.4 Using custom variables

The above are variables supported in the default templates. If you use a custom template, you can use arbitrary variable names following the naming convention (except for body, which is a reserved name and cannot be used in metadata), and the values of variables in the metadata will be passed to your template.

3.3.5 Setting options in YAML

Besides metadata variables, the aforementioned Markdown options (3.1) can also be set in YAML under output -> litedown::*_format -> options, e.g.,

output:
  litedown::html_format:
    options:
      toc: true
      js_highlight:
        package: highlight
        theme: github
        languages: [diff, latex]

3.3.6 Other fields in YAML

See the help page ?litedown::html_format for possible fields in addiction to meta and options that can be specified under the format name, e.g.,

output:
  litedown::latex_format:
    latex_engine: xelatex
    keep_md: true
    template: custom-template.tex

Edit this chapter on GitHub

4 CSS/JS assets

The litedown package aims at lightweight with a minimal number of features at its core, but you can add more features by yourself. In this chapter, we introduce some CSS/JS assets from the GitHub repository https://github.com/yihui/misc.js. You can load arbitrary external JS and CSS files via the js and css meta variables. There are numerous JS libraries and CSS frameworks on the web that you can use, and you do not have to use the ones mentioned in this chapter. You can also write CSS/JS by yourself to enhance your HTML applications.

Remember that the CSS and JS are introduced under the output format litedown::html_format in YAML metadata, e.g.,

---
output:
  litedown::html_format:
    meta:
      css: ["one.css", "two.css"]
      js: ["three.js", "four.js"]
---

For the sake of brevity, we will omit the full YAML fields in examples throughout this chapter but only use the css and js fields. A file name foo.js denotes the file under the js/ directory of the aforementioned yihui/misc.js repository. Similarly, foo.css is under the css/ directory.

All these CSS/JS resources can be used offline, and there are two ways to do it. One way is to clone the GitHub repo to your working directory, and use the files you need, e.g.,

js: ["repo/path/js/callout.js"]

Another way is to enable embedding resources:

output:
  litedown::html_format:
    meta:
      js: ["@callout"]
    options:
      embed_resources: ["https"]

After you have embedded CSS/JS resources once when you have Internet access, these resources will be cached locally and will not require an Internet connection again. This solution works for any online resources, not limited to the yihui/misc.js repository.

4.1 HTML slides

With snap.css and snap.js, you can create lightweight HTML slides:

css: ["@default", "@snap"]
js: ["@snap"]

You can learn more in vignette('slides', package = 'litedown').

4.2 HTML articles

You can style an HTML page in an article format via the following CSS and JS:

css: ["@default", "@article"]
js: ["@sidenotes", "@appendix"]

The article.css is mainly for styling the article frontmatter, body, and side content.

The web version of this book is also based on the article format, so you know what an article format looks like when you read the HTML version of the book.

4.2.1 The overall style

The maximum width of the article body is 800px. For larger screens, this means there will be extra space in the left/right margin, where we can place auxiliary information, such as the TOC and footnotes. On smaller screens, the side content will be collapsed into the body.

The article frontmatter, body, and optionally the appendix are placed in separate boxes.

The default typeface is sans-serif, and you can customize it by supplying an external CSS file (via the css meta variable) or just embedding CSS in the document body, e.g.,

```{css, echo=FALSE}
body {
  font-family: Palatino, "Book Antiqua", Georgia, serif;
  font-size: 1em;
}
```

4.2.2 Side elements

The TOC and footnotes are automatically placed in the margin if space permits. You can also write arbitrary content in the margin via a fenced Div.

4.2.2.1 The TOC

The TOC is sticky on the left side as you scroll down the article. If you do not like this behavior, you may cancel it via CSS:

#TOC {
  top: unset;
}

4.2.2.2 Footnotes

Footnotes are moved to the right side. When you move your cursor over a footnote number in the body, the footnote will be moved next to your cursor. This can be convenient when you have multiple footnotes on a line, since you do not need to look for a specific footnote in the margin.

4.2.2.3 Arbitrary sidenotes

You can write anything in the margin by using a fenced Div with the classes .side and .side-left or .side-right.

Notice

Here is a note on the left side. Anything permitted by law is permitted here. Math? No problem!

$$e^{i\theta}=\sin{\theta}+i\cos{\theta}$$

When you have this sidenote “hammer”, I’m sure you will hit a lot of nails into the margin, even if you do not have to.

::: {.side .side-left}
**Anything** on the left.
:::

::: {.side .side-right}
_Anything_ on the right.
:::

4.2.3 Body elements

Inside the article body, you can write a few special elements.

4.2.3.1 Full-width elements

When an element is wider than the article body, you can show it in its full width by enclosing the element in a fenced Div with the class .fullwidth, e.g.,

::: {.fullwidth}
![text](path/to/image)
:::

Sunspots

If you use R Markdown, you can generate a wide plot or table from an R code chunk, e.g.,

::: {.fullwidth}
```{r}
#| sunspots, echo=FALSE, fig.dim=c(14, 4),
#| fig.cap='Monthly mean relative sunspot numbers from 1749 to 1983.'
par(mar = c(4, 4, .1, .1), bg = 'lightyellow', fg = 'red', las = 1)
plot(sunspots, col = 'red')
grid()
```
:::

If you want to show code (echo = TRUE) but do not want the code to be in the full-width container, you can apply the .fullwidth class to the plot only, e.g.,

```{r}
#| sunspots, fig.dim=c(14, 4), fig.env='.fullwidth .figure .box',
#| fig.cap='Monthly mean relative sunspot numbers from 1749 to 1983.'
par(mar = c(4, 4, .1, .1), bg = 'lightyellow', fg = 'red', las = 1)
plot(sunspots, col = 'red')
grid()
```

4.2.3.2 Left/right quotes

Whenever you find that you are on the side of the majority, it is time to pause and reflect.

—Mark Twain

Sometimes you may want to add a quote but do not want it to take the full width in the body. You may use a fenced Div with the class .quote-left or .quote-right.

Despite the class names, the content does not have to be a quote. If you do want a quote, just use the blockquote syntax >, e.g.,

::: {.quote-right}
> This is a boring quote.
>
> ---Someone
:::

4.2.3.3 Margin embedding

You can embed elements on the left or right margin using a fenced Div with the class .embed-left or .embed-right. These elements will float to the left or right and exceed the margin by about 200px, which can save some space in the article body. You can use the extra space to explain the embedded element with text.

mpg cyl disp hp drat wt qsec vs
Mazda RX4 21.0 6 160 110 3.90 2.620 16.46 0
Mazda RX4 Wag 21.0 6 160 110 3.90 2.875 17.02 0
Datsun 710 22.8 4 108 93 3.85 2.320 18.61 1
Hornet 4 Drive 21.4 6 258 110 3.08 3.215 19.44 1

We have embedded a table of the first 4 rows of the mtcars data on the right margin, which you can see if the browser window is wide enough.

4.3 Tabbed sections

You can load the script tabsets.js and CSS tabsets.css to create tabsets from sections (see documentation here).

css: ["@tabsets"]
js: ["@tabsets"]

4.4 Code folding

Code folding is supported by fold-details.js (see documentation here).

js: ["@fold-details"]

4.5 Callout blocks

A callout block is a fenced Div with the class name callout-*. Callouts require callout.css and callout.js:

css: ["@callout"]
js: ["@callout"]

For example:

::: callout-tip
This is a tip.

> You can write arbitrary content, such as a blockquote.

You can even _nest another callout_ into this one!

Is that cool?
:::

This is a tip.

You can write arbitrary content, such as a blockquote.

You can even nest another callout into this one!

Is that cool?

4.5.1 Built-in callouts

The stylesheet callout.css supports styling tip, note, caution, warning, important, and example callouts. For example:

Thank you for your notice! Your notice has been noted.

Be careful when testing for strict equality of floating point numbers.

seq(0, 1, .2) == c(0, .2, .4, .6, .8, 1)

#> [1]  TRUE  TRUE  TRUE FALSE  TRUE  TRUE

Never try to out-stubborn a cat!

For the sake of reproducibility, please remember to render an R Markdown document in a new R session before publishing or submitting the output document.

::: {.callout-example data-legend="Demo"}
Change the title of this example to "Demo".
:::

Change the title of this example to “Demo”.

4.5.2 Customizing callouts

You do not have to use callout.css but can define your own CSS rules, e.g.,

.callout-important {
  background: red;
  color: yellow;
}

Under the hood, callout.js turns the fenced Div into a <fieldset> for the form:

<fieldset class="callout-*">
  <legend>Title</legend>

  Content.
</fieldset>

The content comes from the original fenced Div. The title comes from the class name (converted to uppercase) by default. You can provide a custom title via the data-legend attribute of the Div, e.g.,

::: {.callout-tip data-legend="Information"}
:::

The icons before the callout titles can be defined via CSS, e.g., you can add two exclamation marks before the title of important callouts:

.callout-important legend::before {
  content: "!! ";
}

The default icons defined in callout.css are essentially UTF-8 characters. In theory, there are hundreds of thousands of characters that you can choose from. Each character is 1 to 4 bytes. For example, you can define a music callout with the music note symbol ♫ in the CSS:

.callout-music {
  background: springgreen;
  border-color: yellow;
}
.callout-music legend::before {
  content: "♫ ";
}

Then you can insert a music callout in your document:

::: callout-music
Please listen to this lovely song.
:::

Alternatively, if you are using callout.css, you can also use CSS variables to define the border color, background, and icon of a callout, e.g.,

.callout-music {
  --callout-background: springgreen;
  --callout-border: yellow;
  --callout-icon: "♫ ";
}

You can use the script right-quote.js to right-align a blockquote footer if it starts with an em-dash (---).

js: ["@right-quote"]

The CSS is necessary only if you want to hide the anchors by default and reveal them on hover.

css: ["@heading-anchor"]
js: ["@heading-anchor"]

4.8 Style keyboard shortcuts

The script key-button.js identifies keys and the CSS styles them, which can be useful for showing keyboard shortcuts.

css: ["@key-buttons"]
js: ["@key-buttons"]

Of course, you can combine any number of JS scripts and CSS files if you want multiple features.

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5 Authoring

5.1 The Knit button

If you use the RStudio IDE, the Knit button can render R Markdown to a litedown output format specified in YAML (e.g., litedown::html_format or litedown::latex_format). Please also remember to add a top-level setting knit: litedown:::knit in YAML, otherwise litedown will throw an error as a reminder.

---
output:
  litedown::html_format: null
  litedown::latex_format: null
knit: litedown:::knit
---

5.2 Live preview

Unless it has become your muscle memory to click on the Knit button in RStudio, you may try to switch to litedown::roam() to preview your HTML output. Note that the preview takes place in memory only. Although you see an HTML page rendered from a file in the preview, the page is not rendered to disk, unless you click on the render button (5.2.3), or call litedown::fuse() (1) or mark() (3) on the file.

5.2.1 Live reload

By default, the preview will automatically refresh the content after you edit and save a file. If you prefer building the document only when you want to, you can turn off the live preview via litedown::roam(live = FALSE). In this case, the document will be rebuilt only when you refresh the page by yourself.

5.2.2 The file listing

After launching the preview via litedown::roam(), a file listing will be displayed, which shows the first few lines of *.R, *.Rmd, and *.md files in boxes, followed by the list of other files.

By clicking on plain-text files, you will see their full content. For binary files, they may be opened in your browser; if they cannot be opened, the browser may prompt to download the file.

Each file will have its size displayed after the filename, with a link attached to the file size. The link points to the raw file (the behavior of the link is up to the browser—the file may be opened or downloaded), and litedown will not process it at all.

The full path of the file or directory being previewed is displayed at the top left.

For .R, .Rmd, and .md files, you can click on the “Run” button (⏵) to render them to HTML in memory and preview the output. This button is displayed at the top right of each file box on the listing page, and at the top right of the preview page of an individual file.

Rendering .R and .Rmd files via the Run button means the full R code in them will be executed. If the R code involves intensive computing, it may not be a good idea to run the whole file, unless you have taken measures to speed up the computing (e.g., via caching).

5.2.3 The buttons

There is a button group at the top right of the preview page.

Code blocks in the preview mode will have line numbers automatically added to their left. If you click on a line number, it will bring you to that line in the source document.

Note that the keyboard shortcuts require the page to be currently on focus before they can take effect. This is important when you are viewing a page inside RStudio or other IDEs, because the viewer may not gain focus automatically, and you will have to explicitly click on it.

5.2.4 Cleaning up

Previewing .Rmd and .R files that generate plots will leave *__files/ directories (containing plot files) on disk by default. If you want to clean up such directories when closing or navigating away from the preview page, you may set the option

options(litedown.roam.cleanup = TRUE)

before you run litedown::roam() or in your .Rprofile. Note that it will not clean up a *__files/ directory if it has existed before you preview a file. This is to make sure roam() will not delete existing files by accident. If you are certain that a *__files/ directory can be safely deleted, you can always delete it by hand. After that, roam() will automatically clean it up when you preview the file again.

Please also note that when caching is enabled (via the chunk option cache = TRUE) in the file being previewed, the *__files/ directory will not be cleaned up, because when a code chunk is cached, it will not be re-evaluated or re-generate plots next time (unless the cache is invalidated).

5.3 Visual editor

Since the Markdown syntax of litedown can be viewed as a subset of Pandoc’s Markdown, you can use RStudio’s visual Markdown editor to author documents. Please bear in mind that most common, but not all, Markdown features are supported.

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6 Books and Websites

Books and websites are usually based on multiple input files under a directory. For a directory to be recognized as a book or website project, it needs to contain a configuration file named _litedown.yml.

If you want to customize the output formats html_format or latex_format for books or websites, you should do it in _litedown.yml, e.g.,

output:
  litedown::html_format:
    options:
      toc:
        depth: 4
  litedown::latex_format:
    meta:
      documentclass: "book"

6.1 Books

The _litedown.yml file should contain a top-level field named book, which currently supports these options:

book:
  new_session: false
  subdir: false
  pattern: "[.]R?md$"
  chapter_before: "Information before a chapter."
  chapter_after: "This chapter was generated from `$input$`."

You can choose whether to render each input file in a new R session, whether to search subdirectories for input files, the types of input files (e.g., you can use .md or .R files if you want), and additional information to be included before/after each chapter, in which you can use some variables such as $input$, which is the path of each input file.

6.1.1 R package documentation

R package developers can build the full package documentation as a book. A series of helper functions have been provided in litedown to get various information about the package, such as the package description (pkg_desc()), news (pkg_news()), citation (pkg_citation()), source code (pkg_code()), and all manual pages (pkg_manual()). You can call these functions in code chunks to print out the desired information. For example, you may put them in the appendix (2.2.6):

# Appendix {.appendix}

# Package Metadata

```{r, echo=FALSE}
litedown::pkg_desc()
```

To cite the package:

```{r, echo=FALSE}
litedown::pkg_citation()
```

# News

```{r, echo=FALSE}
litedown::pkg_news(recent = 0)  # show full news
```

# Manual pages

```{r, echo=FALSE}
litedown::pkg_manual()
```

# Source code

```{r, echo=FALSE}
litedown::pkg_code()
```

6.2 Websites

The _litedown.yml file should contain a top-level field named site, and you are likely to customize the meta variables css, include_before, and include_after for the html_format, e.g.,

site:
  rebuild: "outdated"
  pattern: "[.]R?md$"

output:
  litedown::html_format:
    meta:
      css: ["@default"]
      include_before: "[Home](/) [About](/about.html)"
      include_after: "&copy; 2024 | [Edit]($input$)"

Basically, include_before can take a file or text input that will be used as the header of each web page, and include_after will be the footer.

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Appendix

A For rmarkdown Users

The litedown package has also provided two internal output formats for compatibility with rmarkdown: litedown:::html_document and litedown:::pdf_document.6 The purpose is to make it a little easier to switch from rmarkdown to litedown by mapping some rmarkdown output format options to litedown.

For example, for an R Markdown document with the following output format:

output:
  html_document:
    toc: true
    number_sections: true
    anchor_sections: true
    self_contained: false

You can switch to litedown simply by changing the output format name from html_document to litedown:::html_document. Internally, the above output format is transformed to:

output:
  litedown::html_format:
    options:
      toc: true
      number_sections: true
      embed_resources: false
    meta:
      css: ["default", "@heading-anchor"]
      js: ["@heading-anchor"]

Note that not all rmarkdown options are supported, and not even all supported options have exactly the same effects in litedown. The supported options include: toc, toc_depth, number_sections, anchor_sections, code_folding, self_contained, math_method, css, and includes.

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B Package vignettes

To build package vignettes with litedown, first add this to the package DESCRIPTION file:

VignetteBuilder: litedown

Then use the vignette engine litedown::vignette in the YAML metadata of a .Rmd or .md vignette file:

vignette: >
  %\VignetteEngine{litedown::vignette}
  %\VignetteIndexEntry{Your vignette title}
  %\VignetteEncoding{UTF-8}

The output format of a vignette can be specified in the output field of the YAML metadata, e.g., litedown::html_format (for HTML vignettes) or litedown::latex_format (for PDF vignettes). If no output format is specified, the default is HTML.

The vignette file can be either .Rmd or .md. The former is processed by litedown::fuse(), and the latter is converted by litedown::mark(). Please avoid using the same base filename for two .Rmd and .md files, otherwise their output files will overwrite each other.

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C Technical Notes

C.1 Embedding resources

When https resources needs to be embedded (via the embed_resources option), only these elements are considered:

<img src="..." />
<link rel="stylesheet" href="...">
<script src="..."></script>

Background images set in the attribute style="background-image: url(...)" are also considered. If an external CSS file contains url() resources, these resources will also be downloaded and embedded.

C.2 CSS for margin content

It’s quite simple to move an element into the margin using CSS. For example, the .side-right class in this article is roughly defined as:

.side-right {
  width: 200px;
  float: right;
  margin-right: -200px
}

That basically means the width of the element is 200px and it floats to the right. Now its right side will touch the right margin of its parent element (the article body). What we need to do next is move it further to the right by 200px (i.e., its width), which is done by the -200px right margin. Remember, a positive right margin in CSS moves an element to the left, and a negative right margin moves it to the right.

  1. At one night, as I was thinking about Pandoc Lua filters, an obvious fact suddenly came to my mind: suppose all I care about is HTML output, then the good old JavaScript can actually play a perfect role of Lua filters, because you can manipulate the DOM arbitrarily with JavaScript. ↩

  2. In fact, xml, man, text, and commonmark output formats are supported (thanks to the commonmark package), but perhaps they are not very useful to average users. ↩

  3. Please note that for links and images, their URLs should not contain spaces. If they do, the URLs must be enclosed in <>, e.g., ![alt](<some dir/a subdir/foo.png>). ↩

  4. If you know C, I’ll truly appreciate it if you could help with the LaTeX implementation in GFM: https://github.com/github/cmark-gfm/issues/314 ↩

  5. The specific number doesn’t matter, as long as it’s a unique footnote number in the document. For example, the first footnote can be [^100] and the second can be [^64]. Eventually they will appear as [1] and [2]. If you use the RStudio visual editor to edit Markdown documents, the footnote numbers will be automatically generated and updated when new footnotes are inserted before existing footnotes. ↩

  6. The triple-colon ::: means these functions are not exported, which is to avoid name conflicts between the two packages. ↩