Follow CSS coding standards when authoring your CSS.
Use valid CSS when possible. As an exception, use vendor-specific prefixes to take advantage of CSS3 features.
Minimize CSS hacks. The obvious exception is browsers-specific support, usually versions of IE. If possible, separate CSS hacks into separate sections or separate files.
All possible HTML elements should be styled by the Theme, both in post/page content and in comment content.
Tables, captions, images, lists, block quotes, et cetera.