How reliable is Internet Explorers browser mode?

Question

I can't seem to find any answers to this:

When i change the browser mode in IE, how much can I rely on it actually rendering the webpage as if it were lets say IE8 when I'm using IE10?

Vondavonni

I would have thought that it would be fairly accurate because when IE is in IE8 mode it identifies itself as IE8 in the useragent. Also, I have noticed that a website that uses HTML5 features that IE8 does not support the page does not render correctly. – Milreis

What are you looking for here? A quantifiable measure of the accuracy? At best I think you'll get anecdotal data. You could try rephrasing as "what are the known differences between IE8 compat mode and real IE8?" or something like that. Anyway, I'd guess that it's 97.63255% accurate. – Mope

@jeffamaphone I was thinking more in the line of, does IE come with different versions of trident, or does it disable new features when going into another mode. But hey 97.63255% is pretty good, thanks a ton for the help! – Vondavonni

It's not shipping each version of IE. Search Google using this term: "site:blogs.msdn.com/b/ie compatability view" and you'll see all the blog posts from the IE team on this topic. There are differences, for example, security fixes that were not in IE8 will be in the IE8 compat mode. – Mope

https://mcmap.net/q/610760/-how-well-does-ie7-8-mode-in-ie9-compare-to-actually-running-ie7-8 – Anderaanderea

Answer 1

I'm seeing mixed results so far; I never tested the IE9 version, so cannot comment if Microsoft have done better job this time round (the kinds of things reported at https://mcmap.net/q/610760/-how-well-does-ie7-8-mode-in-ie9-compare-to-actually-running-ie7-8 are much more specific to CSS than I've been looking at).

Specifically I've installed IE10 on Windows 7 (64-bit), and tested HTML5 features. The selection of Document Mode appears to be much more important than the selection of Browser Mode.

• User agent changes correctly

• The IE detection idiom works

   <!--[if lte IE 9]>
   <script language="javascript">
   var is_ie_9_or_earlier=true;
   &lt;</script>
   &lt;![endif]-->
  

• typeof window.PostMessage: WRONG. this is being returned as "object" in IE7 mode. I believe it is undefined in real IE7. IE8/IE9/IE10 correctly return it as "object".

• typeof new XMLHttpRequest().responseType. This is "undefined" in IE7/IE8/IE9, and "string" in IE10. Which I believe is correct.
• typeof Object.keys. Undefined in IE7/IE8, but "function" in IE9/IE10. Again, I think this is correct.

On the other hand ietester on the same machine was much weirder (javascript not running correctly when it should have).

Recoverable