Excellent module. Simple, "press button - receive bacon", interface which works as advertised.
I use it to display code examples on my tutorials, haven't seen it misbihave. You can highlight in different colors: doctypes, comments, tag delimiting angled brackets, tag names, attribute names, attribute values and even HTML entities.
By giving the constructor one short argument you can turn on line numbers for which there is also a separate highlighting color.
A note of caution, if you are going to use the CSS code conveniently provided to you in eg/html-syntax.css make sure to set background colors or make sure they are shown properly from parent elements. I've ran into so many pages that set only the color and on my "dark" system theme colors show up as "gray on gray" ^_^
Excellent work on the part of the author. I give this module 5 stars