I've come across a few MT blogs in which text has been highlighted with a transparent color, as if done with one of those textbook highlighters. When I view source, the tags are emphasis or strong, which suggests the stylesheet has been modified to change those tags.

Anyway, how do I highlight text in html? Thanks.

You can use the background CSS attribute to specify color.  For example, if you want a yellow background for your strong tags, you can have a CSS style like so:

If you want to highlight inline in HTML, you can do it like so:
which then looks like this:

Here is a yellow background example.