From Seo Wiki - Search Engine Optimization and Programming Languages
Hidden text is computer text that is displayed in such a way as to not be easily readable. Hidden text is most commonly achieved by setting the font color to be the same as the background color, rendering the text invisible unless the user highlights it. An example of some hidden text follows on the next line:
This text is hidden because the font color was changed to white.
Hidden text can serve several purposes. Often web sites use it to disguise spoilers for readers who do not wish to read that text. Hidden text can also be used to hide data from users less Internet-savvy or who are not familiar with a particular website. An example of this practice is to display an FTP password in hidden text to reduce the number of users who are able to access downloads and thereby save bandwidth.
It is also used by websites as a spamdexing technique to fill a page with keywords that a search engine will recognize but are not visible to a visitor. However, Google has taken steps to prevent this by parsing the colour of text as it indexes it and checking to see if it is similar to or the same colour as the background, giving those pages much lower rankings.
Compare with metadata, which is usually also hidden, but is used for different purposes.
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