In addition to CSS style information for your theme, style.css provides details about the Theme in the form of comments. The stylesheet must provide details about the Theme in the form of comments. No two Themes are allowed to have the same details listed in their comment headers, as this will lead to problems in the Theme selection dialog. If you make your own Theme by copying an existing one, make sure you change this information first.
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.