The one concern I generally have with the use of both tags is that it might cause a red flag for spamminess - the search engine might see both tags being used and mistake it for keyword stuffing, which is why I try to make sure I use the same text in both attributes. The search engines seem to be aware that Internet Explorer and Firefox have diametrically opposed implementations, and do take that into consideration. However, abusing this could cause red flags. By using the same text in both attributes, it should tip of the engines, hopefully, to the fact that you are trying to accomodate both browsers.

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