Spamdexing:
Generally, you should NOT submit every single page of your website to search engines. In most cases, you should submit only your home page and perhaps a couple of other very important pages. The rest of your website will be indexed by the search engine's spider naturally after the first page is submitted. Over-submitting your website can get you blocked from being listed!

Invisible/Tiny Text and Keyword Stuffing:
Once a very popular form of inserting tons of keywords and phrases into your pages, this is now considered a BIG no-no even though you'll still see it from time to time! This is achieved by placing very small text at the bottom of a page and/or text the same color as the page's background. This, too, can get your website blocked from search engine listings and all efforts of search engine optimization will be poor. Let your website's content speak for itself!

Use of non-compliant HTML to manipulate relevancy:
Using multiple titles and other techniques which aren't HTML standards compliant, used specifically to raise relevancy. The first 2 in the grey area would be real candidates for this area as well. An example of non-compliant HTML would be using a title that does not reflect the content of the page. A table of HTML standard elements is available here. The links lead to information on proper implementation of elements

Use of CSS (cascading style sheets) to manipulate relevancy:
Using hidden elements (layer or span elements etc) that can't be seen by executing code to reveal them. This activity, to our knowledge, has not been addressed in search engine content guidelines. Layers create problems in the search engine optimization process.

Comments:
Comments help maintain the code in an HTML document and in the search engine optimization process. Comments should not be used to raise relevancy or manipulate search engine descriptions. Previously *on site* in Excite content Guidelines.

Invisible form elements:
Used to hold keyword values, not a well known technique; however they can be used this way. Not known to be mentioned specifically in any search engine content guidelines or "unwritten policy"