David Baron's DOM test suite
Tests that work perfectly so far: DocumentFragment, Document, Comment, Node, Text

more jscript tests

Some pages with tests from mozilla.org:

- tests lots of pages
- collection of links to test pages

Some more CSS: http://haughey.com/csshorrors/
The evil Tests: http://www.hixie.ch/tests/evil/mixed/home.html

The w3c CSS test suite: http://www.w3.org/Style/CSS/Test/

CSS+Javascript tests: http://www.quirksmode.org

More evil CSS tests on http://www.endoframe.com/css/tests/