The Stack Exchange engine will not add syntax highlighting by default. However, you can add explicit language hints like this:
<!-- language: lang-html -->
This is what we've been doing so far, but there is an even better way.
Stack Exchange allows to assign a language hint to every tag. It will then use that language to highlight the code in the question and its answers. If there's more than one tag that has syntax highlighting, it lets the highlighting engine infer what's the best language to use. See more information here:
You can see the assigned language on a tag's information page:
How you can help
Think of you favorite tags and post an answer to this question that will list tag names with desired syntax highlighting.