Recently Matt Cutts of Google fame fired a shot across the bow of website owners everywhere with his post regarding hidden links, different in nature than hidden text. His post sends a clear indication that hidden links is something that the spam team is aware of, and as his points out is a violation of their quality guidelines, which can lead to penalties or full out removal from the index.
A hidden link is a link to another page that is not viewable to the user, but will be crawled by any crawler. There are very complicated ones as explained in Matt’s post above and very simple ones where the anchor text is left out or even a link attached to a small 1×1 clear pixel. The violation is also just as clear, a hidden link can be seen as an attempt to send a search engine crawler on to that page, cast a vote, and lead to indexing, while at the same time not detracting from the users experience on the page.
Some hidden links may have been put there intentionally by the webmaster, perhaps to get a new site crawled. Others may have been put there by a third party, such as a free page counter or statistics script that the site owner cut-n-paste onto their site, unknowingly placing the hidden link.The worst kind is when the site is hacked and the hacker will place hidden links to their spam sites, usually of questionable subject matter. These links may go undetected by the site owner as they only appear in the code, but when viewing the site in a browser the page appears normal.
I use the web developer for extension daily. One of the many features of that tool is under Outline>>Outline Links>>External Links. When this feature is turned on and you are viewing a page any links to an external page, not on the same site, will be outlined in a bright red box. Today while researching another project I came across just such a page. A screen shot is provided to illustrate how it looks when highlighted.
Notice on the bottom left of the page, those four little boxes are actually links that the search engine crawler will follow. No real user is meant to follow these, and I’m not sure what purpose they actually serve, but if detected Google could easily penalize or ban the site for such actions. This particular example uses two different types of hidden links, one with barely visible text, just a period, and the other with a small 1 x 1 pixel picture.
The code for the example shown above looks like this, note the > . < as the anchor text and the image sourced with a zero border.
Viewing your own site periodically with the tools described above may just save you from being banned from Google. If you do find it on your site and remove it, I’d fire off a quick reinclussion request to Google. It’s been affirmed that such requests are for sites that have some partial penalties applied to them as well as full out banned. In the request of course document your findings, the changes you made, and if at all possible the reason you feel the hidden link was there in the first place.