What search engines don't like

Don't do it!
It is extremely important to know what search engines don't want. Otherwise, your perfectly optimized site may not be indexed -- and could even be blacklisted. Therefore, when you learn about factors that influence search engine rankings, you should also learn which tactics to avoid.

Spam - search engines' worst enemy
Search engines hate tactics intended to fool them into awarding high rankings to irrelevant pages. These tactics are called "spam." Search engines strive to provide the most relevant results to their users, but spam clutters their indices with irrelevant information.
Some webmasters create spam after they learn which criteria search engines use to rank pages. For example, search engines give high scores to pages filled with keywords. Webmasters came up with a way to add more keywords without sacrificing a site's appearance. They use invisible text ( the background and the text are the same color, so the text is not seen by the visitor). Previously, robots that indexed invisible keywords ranked those sites higher for keyword frequency and weight.
Search engines now know of this technique and define it as spam. Currently, sites that use invisible text are banned from most of the major search engines.

The following techniques are usually considered spam:

Meta refresh tags
Invisible text and overuse of tiny text
Irrelevant keywords in the title and meta tags
Excessive repetition of keywords
Overuse of mirror sites (same sites that point to different URLs)
Submitting too many pages in one day
Identical or nearly identical pages
Submitting to an inappropriate category (for directories)
Link farms

Frames, dynamic content and Flash intros
Although search engines won't penalize for the use of frames, dynamic content and multimedia files, they will have difficulty indexing them.
Recently, some engines started to index dynamic content. However, most search engines are still unable to index multimedia and dynamic pages, and those that are, don't index all of them. Here's a list of files that search engines don't index:

Text in graphics (use ALT tags)
Pages that require registration, cookies or passwords
XML
Java applets
Acrobat files (PDF), except Google
Dynamic content (URLs with "?" in them), except Google, Altavista, FAST and Inktomi
Multimedia files (Flash, Shockwave, streaming video)

Workaround pages
If your site consists largely of files that search engines don't index, create workaround pages. Workaround pages should contain the most important information about your products or services. Workaround pages should be optimized just like doorway pages. 
Search engines will index workaround pages even if they can't index the rest of a site. Always link to these pages from your site map to make sure search engines spider them, and submit workaround pages rather than non-optimized pages.

An Overview

Mistakes

Don't use spam or frames
Create workaround pages for dynamic and framed sites    

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