Hidden text is text which is embedded in the html code and seen by the search engines but not seen by website visitors. Those that use hidden text believe that it can improve their page’s ranking and in some cases it may well appear to do so but the effect is very small.
It is quite easy to hide text as we shall see but all the major search engines advise strongly against doing so, because it reduces the quality of their results. Search engines can detect certain forms of hidden text algorithmically and when they do they will automatically drop the offending site from their index. It is because the search engines regard hidden text as spam and are getting better at detecting it every day that hiding text is a low reward high risk strategy and one never used by professional SEO’s.
The first kind of hidden text is not so much hidden as ignored. This is text placed between comment tags like this <!–This text is a comment–>. Comment tags are actually intended to be used as an aid to whoever is editing the source code at a later date and as such comments are not of course displayed in the browser. Here is a real life example of comment tag spam:


Obviously the coder was unaware that search engines completely ignore everything that is written between the comment tags.
The commonest form of hidden text is where the coder simply writes text with the same color as the background. In this example from a gambling site they have set the text color to be white (the same as the background) like this <p class=”normal” style=”color: #ffffff;” >spam here</p>. The section of the page concerned naturally looks like this to visitors:

But by highlighting the appropriate area we can see the hidden text.

Some coders believe that instead of making the text color exactly the same as the background color, if they make it very close, for example #eeeeee against a white background #ffffff, then the search engines will not treat it as spam. This is not the case.
It gets a little more sophisticated when the coder hides text in a span or div with the class having the {visibility:hidden;} property. Here is an example from an affiliate printer site where the coder has used this technique:
/* small interval */
#smallinterval {
height: 0px;
}
#smallinterval h1 {
display: none;
}
The top of the page looks normal to visitors but again by highlighting the appropriate area we can see the result of the use of <p class=”cleartext”>, in this case partially obscured by AdSense.

It may not look much from the highlighting but the coder has managed to hide over four thousand words, sixteen pages of keyword rich text! This must be approaching a world record and here is a small sample of the hidden text:

There are many other even more devious methods such as putting the style sheet in a sub-folder and excluding the folder in the robots.txt file or naming the .css file something like style.js and declaring it like this <link rel="stylesheet" href="style.js" type="text/css">
Not quite classic hidden text but certainly text that is not normally seen is this example of a stuffed alt-tag. The coder has used a small graphic to round the bottom corners of their design. Here is the graphic:

And here is what it looks like to the casual visitor when used in the bottom left hand corner of the page:

In fact the coder has used similar graphics in all corners and we can see the text of the stuffed alt tag in our example by holding the cursor over the offending graphic. This is what you see:

The irony for the users of these techniques for hiding text is that if for some reason the page ever did well enough to be in the top positions of the SERPs for their keywords, competitors would report them as spam with Google’s spam report form for example: