Does it also work when the syntax is outside of the diff context? I.e. think a html comment that started in an a prior insertion hunk? We had encountered a lot of edge cases whilst building the syntax highlighted diffs for https://www.tixef.com It is a tricky domain, hats of to you for attempting it in JavaScript!
The highlihght is pretty much constrained to diff (on pull requests, commits and compares[1]), but it could work on any page. I've mentioned[2] how I used jquery-syntaxhighlighter to do it, which itself is a fork of Google's Prettify in order to do the heavy lifting of syntax discovery and actual highlight.
