When someone searches for terms in a comment, those terms could be highlighted with the mark tag in the comment text.

Mockup:

Comment text is stored in comments.comment as the user wrote, using markdown, and rendered to html in comments.markeddown_comment. This is so we don't incur that render cost on every view of a comment, but it means this feature will need to operate on the HTML rather than the markdown. (I suppose it's possible as part of adding Solid Cache in #1366 that we could move this functionality out of the database and into a fragment cache with a long expiry, say 365 days. Though we'd want to add significant testing that changes to our markdown code or commonmark don't lead to comment rendering changing meaningfully over time!)

A corner case: MariaDB's full-text indexes for search ignore punctuation, whether used as part of a sentence or as markdown. So you can search for a quoted phrase like "sometimes correctly" to find this same comment where it is rendered in HTML as correctly.</p></blockquote><p>Sometimes.