You can use em values to size tables, infoboxes and some other stuff, but not for images. This is crucial as skins can differ their base font size (13, 14, 16 px) and images in em-sized infoboxes then overflow.

See also T247682 for alternative solution.

Currently, images on MediaWiki pages can only be resized using pixels (px). This limitation can be problematic for in-line images, especially for those meant to resemble or replace text characters that are unavailable in Unicode (e.g., unique glyphs like a question comma or Prince's "Love Symbol" glyph). Additionally, some templates, like [[template:nospam]], use images to obscure characters (such as @) to prevent email-harvesting bots from recognizing email addresses.

When an image is used in-line, it doesn't adjust its size to match surrounding font changes. For instance, it remains the same size when page font is adjusted, or if <big> or <small> tags are used, which can disrupt visual coherence.

However, HTML embedded with CSS does allow an image's size to be set using em units, which scales the image relative to the surrounding text's font size. One em is the width of a letter M. Implementing support for em-based image scaling within MediaWiki would enable in-line images to grow or shrink in tandem with text, improving consistency across varying font sizes.

It's usual for such sizing to be quite small, e.g. 0.35em

Here's a quick-and-dirty example of how this can be implemented in html:

<img src="character.png" style="width: 0.5em; height: auto;" alt="Character representation">

and how this could be called in wikitext:

[[image:character.png|0.5em|character representation]]

Benefits:

• Provides greater flexibility and consistency for in-line image usage.
• Enhances readability and visual alignment in text-heavy contexts where the image should match text size (such as in academic or linguistic content).
• Offers a valuable tool for templates or icons embedded in text that need to adjust dynamically to the viewer's preferred font settings.

Notes

The file that should be changed is includes/media/MediaTransformOutput.php.