Flamebait title, please do not do that.

What are we looking at?

This is not a flamebait title. It explicitly declares what Microsoft is doing. They have a list of sites where IE9 is specifically being told to render in a different compatibility mode than what the owners of those sites have indicated.

This isn't really Microsoft- or IE-specific. Opera does this too:

http://www.opera.com/docs/browserjs/

Chrome also added special handling to work around poor browser sniffing in Hotmail, which is ironic because Opera had to do the same for Google Docs. Opera claims that most of what they have to deal with is actually improper browser sniffing on sites that didn't work in a previous version of Opera but work fine now. Looking at Microsoft's list of sites, it seems to be a similar story, many sites have IE-specific code that isn't necessary in IE9.

I like how they finish strong with zune.net

For people who want more information about what particular elements/attributes mean, there's http://msdn.microsoft.com/en-us/library/gg699485(v=vs.85).as...

There are also steps on that page for getting your site off of the compatibility list.

They should try and get live.com out of there too :)

Well Microsoft is not necessarily breaking, people have special casing in their sites for older browsers that they might want to evaluate based on what browser mix is hitting them.

The list looks quite small though. I was expecting it to be gigantic list in thousands.