Make HTML Elements Full-width Automatically via JavaScript and CSS

Yihui Xie 2023-09-14

I mentioned a simple CSS trick to generate full-width figures in 2021. I was thinking the other day if I could make certainly elements automatic, so I do not need to manually specify them to be full-width. Finally, I wrote some JS code to deal with three types of elements automatically:

  1. Code blocks (<pre><code>).

    If the scrollWidth is greater than offsetWidth, it means the code block has a horizontal scrollbar, and we may want to make it full-width.

  2. Tables (<table>).

    If its offsetWidth is greater than its parent’s offsetWidth, it is too wide.

  3. Table of contents (an element that has the ID TableOfContents, e.g., <nav id="TableOfContents">).

    If any TOC item has multiple rectangles on the layout (getClientRects().length > 1), it means the item is wrapped, and the TOC may benefit from more space.

The JS code will add a class name fullwidth to elements that need more space. Users can define CSS rules for this class, e.g.,

.fullwidth {
  width: 100vw;
  margin-left: calc(50% - 50vw);

On my website, I do not really use the full screen width, but only the body width. It’s up to you how wide you want your .fullwidth elements to be. You can load the JS from the jsdelivr CDN:

<script src="" defer></script>

The above code block is also an example of an automatic full-width element. If you view it on a large screen such as your computer screen, it should take the full body width.