There's a lot of debate in the web design/development/standards world at the moment about a proposed new feature in Internet Explorer 8 as described in this issue of A List Apart and, in particular, in this article about a browser rendering switch using a meta tag and this article on the IE Blog.
There are proponents and opponents of this type of meta tag functionality. I don't know that I am knowledgeable enough to join the debate here. However, in the main, I am not sure that I see the need for the new meta tag browser rendering functionality since the advent of IE7 with its better CSS support. There was a big step forward with the IE6 to IE7 release and I imagined that the 'problem' of websites 'breaking' was not likely to be as big with the IE7 to IE8 transition. My experience of websites 'breaking' after the IE7 release was fairly limited. Apparently, that's not the case says the IE Blog. I'm not convinced though.
The default behaviour of the new proposed meta tag seems a bit daft as well (as explained in this article by Jeremy Keith). Unless you specify it, the default behaviour of IE8 will be to render in IE7 mode. Surely the default behaviour should be the current version of the browser? Not the last version?!
I hope common sense prevails. Use a meta tag if you must but make the default behaviour more sensible.
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