Keyword density is the ratio of the number of occurrences of a particular keyword or phrase to the total number of words in a page. The total number of words does not include only the words in your title, description, or even in the body. But it also includes Title, Meta Description, Meta Keywords, Visible Text, Alt Tags, Comment Tags, Domain Name, Image tags, Linked Text, Option Tags, and Reference Tags.
Keyword density is not constant for all Search engines. Simply every search engine algorithm differs, and for that you must choose an average keyword density that make your site rank in the top ten position in all search engines. If we make a research we will find that the best keyword density for Google is about 10%, Yahoo is about 20% and Msn is about 15%. Then you must make your site keyword density about 15% in order to get huge traffic to your site from those major search engines. It's up to you, you may choose to optimize your site for just one of search engines, or to target all the major search engines.
In conclusion, you must balance between Off-Site-Optimization and On-Site-Optimization. Also you should give some attention to keyword density in order to make successful site, and you can decide whatever you want to target all the search engine or a particular search engine.
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