Most people understand hidden text is something like white text on a white background, and know to steer clear of it. Let me show you an example of a hidden link. Normally a hidden link could be in several forms:

– hidden text that also happens to be hyperlinked, e.g. white text on a white background, and the text is a link

– using CSS to make hyperlinks that are tiny, like 1 pixel high text

– hiding links in something like the period in the middle of a paragraph of text

Now there’s nothing bad about changing the style of a link to some degree, but let me show you an example of going overboard. Here’s a paragraph of text on a site that I recently saw:

You see the two normal hyperlinks, right? Do you see any other links in this paragraph? A user wouldn’t see any other links, even if they moused over every word in the paragraph. But if you happened to click on just the right word, you’d get whisked away to a hardcore porn site. Here, I’ll show you what you’d see in the instant after clicking on the hidden link, right before you head to the porn site:

See how the word “mission” has a little box around it? It’s a hidden link. If you view the source of the page, here’s what you’ll see. I’ve highlighted the relevant link:

Someone went to a fair amount of trouble to hide the porn site link. The status bar gets set to empty using the onMouseOver action, so when you mouse over the link, you don’t see that it goes anywhere. And the style of the link is set so that the cursor doesn’t change when you mouse over the link as well. In my opinion, this is a good example of a link that crosses over into deceptiveness and violates our quality guidelines.

As long as we’re talking about links, this seems like a pretty good opportunity to talk about a simple litmus test for paid links and how to tell if a paid link violates search engines’ quality guidelines. If you want to sell a link, you should at least provide machine-readable disclosure for paid links by making your link in a way that doesn’t affect search engines. There’s a ton of ways to do that. For example, you could make a paid link go through a redirect where the redirect url is robot’ed out using robots.txt. You could also use the rel=nofollow attribute. I’ve said as much many times before, but I wanted to give a heads-up because Google is going to be looking at paid links more closely in the future.

The other best practice I’d advise is to provide human readable disclosure that a link/review/article is paid. You could put a badge on your site to disclose that some links, posts, or reviews are paid, but including the disclosure on a per-post level would better. Even something as simple as “This is a paid review” fulfills the human-readable aspect of disclosing a paid article. Google’s quality guidelines are more concerned with the machine-readable aspect of disclosing paid links/posts, but the Federal Trade Commission has said that human-readable disclosure is important too:

“The petition to us did raise a question about compliance with the FTC act,” said Mary K. Engle, FTC associate director for advertising practices. “We wanted to make clear . . . if you’re being paid, you should disclose that.”

To make sure that you’re in good shape, go with both human-readable disclosure and machine-readable disclosure, using any of the methods I mentioned above.