' character entity not compatible with older HTML
A debian user writes [1]
highlight 3.41 uses "'" for HTML output which only appears
in HTML 5.0 and XHTML. This causes problems with older browsers
such as w3m which haven't been updated, yet (separate bug report
just filed).
I suggest using the "'" entity, instead; which works with
all versions of HTML.
[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=927410