By the standard, text that should be displayed on mouseover should be in the title attribute, and text that should be displayed when the image is not available, or which should be shown on non-visual browsers, belongs in the alt attribute. However, Internet Explorer (and then most other browsers) does the opposite, displaying what is in the alt attribute on mouseover. Firefox broke this trend with version 2 (if I recall correctly) as they moved to be more standards compliant. Now, browsers may use either attribute. Personally, I use both on my sites, where appropriate. However, when doing so, I put identical text in both attributes, to avoid the search engines red flagging the page.

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The best way to learn anything, is to question everything.