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The idea was to incorporate this idea into Internet Explorer but was met with webmaster opposition. So far, it has not happened.
Word and Excel use a simple verion. MS were going to sell words to companies and use these bought words as SmartTags with the dotted underline by directing traffic to the advertisers site.
This tag was to stop this from happening. It was out-out by the webmaster (at least you could opt out).
Some webmasters leave this tag in in case MS do decide to release this in future versions of their browsers. To have this tag or to miss it out makes no difference at the moment.