Internet Explorer 9 Facets: IE9, IE8, IE7 and IE6

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Internet Explorer 9 has been optimized so it won’t break the web, just as other Internet Explorer releases before it, including IE8. The level of optimization poured into IE9 in terms of backward compatibility involves building the browser so that it will seamlessly render different markup, especially markup that wasn’t tailored to the next generation of IE, without users noticing any difference in the way they experience favorite websites, even though the browser version has changed. As far as Internet Explorer 9 is concerned, this is done through ensuring that the browser sports a number of different modes.

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Internet Explorer 9 Facets: IE9, IE8, IE7 and IE6 - There are four different modes - Softpedia