Quickie CSS Test:
If this sentence is not rendered with green text, your browser probably does not support CSS.
There are already some excellent CSS tests available online:
W3C's CSS1 Test Suite
Ian Hickson's Evil Tests
These are primarily CSS-intensive tests, rather than HTML-intensive tests, so they should be scored separately from the HTML tests. However, many of these HTML 4.01 tests use stylesheets to make it clearer whether certain elements are supported, so elements that would not automatically look different from normal text can be noticed.