GOOGLE:Quality guidelines :1

Hidden text and links
Hiding text or links in your content can cause your site to be perceived as untrustworthy since it presents information to search engines differently than to visitors. Text (such as excessive keywords) can be hidden in several ways, including:
Using white text on a white background
Including text behind an image
Using CSS to hide text
Setting the font size to 0
Hidden links are links that are intended to be crawled by Googlebot, but are unreadable to humans because:
The link consists of hidden text (for example, the text color and background color are identical).
CSS has been used to make tiny hyperlinks, as little as one pixel high.
The link is hidden in a small character - for example, a hyphen in the middle of a paragraph.
If your site is perceived to contain hidden text and links that are deceptive in intent, your site may be removed from the Google index, and will not appear in search results pages.

When evaluating your site to see if it includes hidden text or links, look for anything that's not easily viewable by visitors of your site. Are any text or links there solely for search engines rather than visitors?
If you're using text to try to describe something search engines can't access - for example, Javascript, images, or Flash files - remember that many human visitors using screen readers, mobile browsers, browsers without plug-ins, and slow connections will not be able to view that content either. Using descriptive text for these items will improve the accessibility of your site. You can test accessibility by turning off Javascript, Flash, and images in your browser, or by using a text-only browser such as Lynx. Some tips on making your site accessible include:
Images: Use the alt attribute to provide descriptive text. In addition, we recommend using a human-readable caption and descriptive text around the image.
Javascript: Place the same content from the Javascript in a no script tag. If you use this method, ensure the contents are exactly same as what is contained in the Javascript and that this content is shown to visitors who do not have Javascript enabled in their browser.
Flash: Consider placing descriptive text and site navigation outside of Flash.
Videos: Include descriptive text about the video in HTML. You might also consider providing transcripts.

If you do find hidden text or links on your site, either remove them or, if they are relevant for your site's visitors, make them easily viewable. If your site has been removed from our search results, review our webmaster guidelines for more information. Once you've made your changes and are confident that your site no longer violates our guidelines, submit your site for reconsideration.

If you'd like to discuss this with Google, or have ideas for how we can better communicate with you about it, please post in our webmaster discussion forum.

Cloaking, sneaky Javascript redirects, and doorway pages
Cloaking
Cloaking refers to the practice of presenting different content or URLs to users and search engines. Serving up different results based on user agent may cause your site to be perceived as deceptive and removed from the Google index.
Some examples of cloaking include:
Serving a page of HTML text to search engines, while showing a page of images or Flash to users.
Serving different content to search engines than to users.
If your site contains elements that aren't crawlable by search engines (such as Flash, Javascript, or images), you shouldn't provide cloaked content to search engines. Rather, you should consider visitors to your site who are unable to view these elements as well. For instance:
Provide alt text that describes images for visitors with screen readers or images turned off in their browsers.

Provide the textual contents of Javascript in a noscript tag.
Ensure that you provide the same content in both elements (for instance, provide the same text in the Javascript as in the noscript tag). Including substantially different content in the alternate element may cause Google to take action on the site.Sneaky Javascript redirects
When Googlebot indexes a page containing Javascript, it will index that page but it cannot follow or index any links hidden in the Javascript itself. Use of Javascript is an entirely legitimate web practice. However, use of Javascript with the intent to deceive search engines is not.

For instance, placing different text in Javascript than in a noscript tag violates our webmaster guidelines because it displays different content for users (who see the Javascript-based text) than for search engines (which see the noscript-based text). Along those lines, it violates the webmaster guidelines to embed a link in Javascript that redirects the user to a different page with the intent to show the user a different page than the search engine sees. When a redirect link is embedded in Javascript, the search engine indexes the original page rather than following the link, whereas users are taken to the redirect target. Like cloaking, this practice is deceptive because it displays different content to users and to Googlebot, and can take a visitor somewhere other than where they intended to go.

Note that placement of links within Javascript is alone not deceptive. When examining Javascript on your site to ensure your site adheres to our guidelines, consider the intent.
Keep in mind that since search engines generally can't access the contents of Javascript, legitimate links within Javascript will likely be inaccessible to them (as well as to visitors without Javascript-enabled browsers). You might instead keep links outside of Javascript or replicate them in a noscript tag.

Doorway pages
Doorway pages are pages specifically made for search engines. Doorway pages contain many links - often several hundred - that are of little to no use to the visitor, and do not contain valuable content. HTML sitemaps are a valuable resource for your visitors, but ensure that these pages of links are easy for your visitors to navigate. If you have a number of links to include, consider organizing them into categories or into multiple pages. But in doing so, ensure that they are intended for visitors to navigate the sections of your site, and not simply for search engines.

Google's aim is to give our users the most valuable and relevant search results. Therefore, we frown on practices that are designed to manipulate search engines and deceive users by directing them to sites other than the ones they selected and that provide content solely for the benefit of search engines. Sites making use of these practices may be removed from the Google index, and will not appear in Google search results.

If your site has been removed from our search results, review our webmaster guidelines for more information. Once you've made your changes and are confident that your site no longer violates our guidelines, submit your site for reconsideration.

If you'd like to discuss this with Google, or have ideas for how we can better communicate with you about it, please post in our webmaster discussion forum.