One of the most common instances of spam on websites is the use of hidden keywords, which is also commonly referred to as invisible text. Hidden keywords are defined as text that cannot be seen by regular visitors but that can be seen by robots and spiders. The theory behind hidden keywords is that search engines will index the words that are invisible, thus making the page more keyword dense.
There are two ways to incorporate hidden keywords: through HTML and through CSS (Cascading Style Sheets). Hidden keywords in regular HTML can be achieved by specifying a font color for block text (which is usually "stuffed" with keywords) that's the same as the background color, thereby making it "disappear" into the background. This form of invisible text is easily detectable by search engines. The use of hidden keywords though CSS is more complicated in that the color of the text is defined in an external file, which is not crawlable by search engines.
Avoiding Hidden Keywords
Although almost all forms of hidden keywords will produce good results in the SHORT term, all major engines have implemented filters that are capable of detecting this type of spam. Websites found to be using hidden keywords will almost certainly be permanently removed from the index of all search engines. For this reason, we encourage you to refrain from using hidden keywords on any of your webpages.