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

If you are talking about creating new web pages for use on the public Internet, my advice would be to use an HTML5 doctype or other strict doctype and no X-UA-Compatible meta tag. This would be equivalent to one with IE=edge, by the way. This is for web pages compatible with all versions of IE.

You can add the X-UA-Compatible meta tag to an existing page that is only compatible with IEn and lower if you don't have the time or inclination to update it to support the latest version(s) of IE right now. In that case you can use IE=n, where n is the latest version of IE that the page works on. IE versions later than n will go into "IEn mode" and render the page as IEn did. Note that n cannot be less than 7, as that is the first version to support this system. There is no real point to using IE=EmulateIEn on an actual page, that is only useful for when you are sending a blanket header to an entire site via http, instead of altering individual pages. In that case, EmulateIEn would go into "IEn mode" only if a strict doctype was on the page, and into "quirks mode" otherwise.