IE8 browser mode vs document mode

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From this article on the IE8 blog, entitled How IE8 Determines Document Mode

  1. The Developer Tools settings override all Document Modes for pages displayed in a tab.
  2. The X-UA-Compatible meta tag and then header override Compatibility View Settings and the doctype unless the X-UA-Compatible value is EmulateIE7 or EmulateIE8.
  3. The user’s Compatibility View Settings override the Microsoft Compatibility View List.
  4. If none of the above rules apply, the doctype determines whether the webpage renders in IE8 Standards, IE8 Almost Standards or Quirks Mode.

So from that we get the following answers to your questions:

Q. What is the difference between browser mode and document mode in simple terms?
A. Browser mode is set in the developer tools to emulate different IE browser version behaviors while document mode is defined on the web page to tell IE to render the site differently for compatibility purposes.

Q. What causes the browser mode to change?
A. The user changes the browser mode in the dev tools.

Q. What causes the document mode to change?
A. The Doctype and the X-UA-Compatible meta tag and header set by the web developer.

Q. If a user changes the mode(s) via developer tools, does the change remain even if the page is refreshed?
A. The Browser Mode will stay, but if you change the Doctype and X-UA-Compatible, they will go back to what is defined on the page.

UPDATE: As Adrien Be points out below, IE9+ adds the ability to change the document mode in the dev tools via a setting which will persist on refresh.

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