W3C CSS1 Test Suite

Related Resources

Historical W3C CSS tests
A hodgepodge of tests and examples, some of which made their way into the official Test Suite.
NIST CSS test pages
Neither for the faint of heart nor the impatient of mind, these tests are nonetheless an amazing resource.
CSS Pointers Group
A good collection of links to tips, tricks, tutorials, and other resources, all managed by CSS mavens Sue Sims and Jan Roland Eriksson.
The primary newsgroup for stylesheet authors. If the residents here don't know the answer, you've probably found something new. Note: in order for this link to work, you'll need a newsreader with access to a server that carries the group.