Which X-UA-Compatible mode should I be using?

Question

I have been reading around the subject of using document compatibility meta tag to web pages.

Answer 1

What Rob said. Supporting various IEs has nothing to do with compat mode. The X-UA-Compatible header simply makes the IE the site is viewed on to use the latest, most modern layout engine that it has if you set content="IE=edge" IE9 has all IE 8, IE 7, IE 6, and quirks mode supported. IE10 will be supporting IE9 and below as respective engines to render on. So, if you set content="IE=6" it means that particular IE is supposed to use IE 6 engine to render the site - which completely defeats the purpose of using a modern browser.

In HTML5 Boilerplate, we also add chrome=1 so if Chrome Frame is installed, that site will take advantage of that and render the site like it is rendering on Chrome.