Microsoft has announced that IE8 will introduce a new “browser version targeting” system that will see the browser default to rendering standards-compliant pages the same as in IE7, and will require developers to explicitly opt in to IE8’s new, Acid2-compliant rendering engine if they want it.
Furthermore, since Microsoft believes this is the best way for all browsers to move the Web forward without breaking current sites, it has implemented this opt-in mechanism in a form that all browsers can support:

When placed in the head of an HTML page, this tag will tell IE8 or later to render the page using the most advanced (and standards-compliant) rendering engine available in IE8, instead of the default, IE7-level rendering mode.
In the future, other browsers may implement this standard too, if they find it useful (more on this below):

If you wish to avoid adding a tag to all of your pages, you can instead configure your web server to send the same information as an HTTP response header, though if history is any guide, the tag will be by far the preferred mechanism.
And if you prefer the current browser behavior of always using the most standards compliant rendering available for the document’s DOCTYPE, you can use the special edge value to request that:

But Microsoft believes this is not what most people will want if they stop and think about it. Mainly, this feature is there for experimental/demo pages (such as the Acid2 test).
By requiring developers to declare when they want new browser rendering engines to kick in, Microsoft hopes to eliminate the kinds of page layout issues that plagued the release of IE7. From now on, if you write a page for a particular version of IE, it should work in all future versions of IE.
This move, if embraced by developers, would represent a fundamental shift in the role of standards on the Web. Understandably, quite a few people are upset by this”