Search Engines
Many webmasters reported that Google's spider started to index their external CSS files. What are CSS files, why does Google index them and how can this affect your rankings on Google and other search engines?

What are CSS files?
CSS (Cascading Style Sheets) files allow webmasters to specify the layout of a web site without touching the HTML code. A good example is the CSS Zen Garden site.

On that web site, you can change the complete look of the site by selecting different style sheets. The HTML code of the page is always the same, only the CSS file that is used to display the HTML code changes.

What's the problem with CSS files?
CSS allow webmasters to design their web pages without touching the HTML code. That also means that webmasters can use that technique to hide content from their web site visitors.

Webmasters can put keyword rich content in the HTML code of their web pages that search engines can index. The same text can be hidden from human web surfers by using CSS.

That means that search engines see something totally different than human web surfers. Unfortunately, some webmasters try to fool search engines with that technique.

What do search engines do about this problem?
Search engines don't want to be fooled by webmasters. For that reason, they started to index CSS files. If search engines find anything that looks like spamming in a CSS file, they might penalize the corresponding site.

What can you do to avoid problems with your CSS files?
Unfortunately, it's not clear yet what search engines consider spamming and what not. There are many legitimate uses for hiding text on a web site so it's hard to tell what search engines will penalize.

As a rule of thumb, don't try to cheat search engines. If you do something explicitly to cheat search engines then chances are that search engines will detect this and penalize your site sooner or later.

Ask yourself if it is really necessary to hide text on your web site. If it might be misinterpreted as spamming by human web surfers, don't hide text on your site. Don't use shady techniques to promote your web site.