When will my site's CSS affect my search engine ranking?

At first I thought css was used for absolutely nothing but styling the document when the user viewed it in a browser. But then I realized that css is also used by search engines in indexing pages. Search engines don't index content with display: hidden I believe, and heavily penalize sites that use keyword stacking (text that is never seen by the user, either the same color as its background or that appears f开发者_StackOverflow社区ar off the screen).

Are my 2 assumptions correct? How else is CSS used by search engines? Specific examples of CSS that would change my search engine ranking would be awesome. :D

In general, your assumptions are correct. If the text cannot be seen by the user, then it is not meaningful text, and therefore it is not indexed. In the past, webmasters would game the system by putting irrelevant keywords and text in their pages and hiding it, in an attempt to get illegitimate search engine hits.

In addition, use of headings such as H1 and the positioning of the text on the page (which can be affected by CSS) has an influence on search ratings.

Yes you are right. Do a quick search on google with the keywords SEO and CSS and you'll find plenty of articles like this one:

http://www.stonetemple.com/articles/css-and-seo.shtml