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[Content] Attributes [Action] 
In addition to indexing plain text surrounded by tags, search engines should generally index certain selected tag attribute text. The most important is the ALT attribute in the IMG tag, which contains a textual description of the image linked into the page -- an excellent piece of additional data. Much more rare, but still useful, is the LONGDESC attribute, which is a link to a page containing more information about the image or object: you may want to index this page separately, or incorporate it into the contents of the linking page.
Indexing Robot Crawler Checklist - SearchTools.com
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