Prior to the deprecation of the 'X-' format (see RFC 6648) any X- prefixed header is an eXperimental or non-standard additional header. In this case it would seem that some application is reporting the version of flash. After upgrading from Version 12.2 of the Citrix online plug-in to Version 3.x of Citrix Receiver for Windows, proxy connections to external Web sites might fail to launch with NTLM proxy authentication enabled. From RcvrForWin4.114.1.100#LA3781.

October 2015 UpdateThis answer was posted several years ago and now the question really should be should you even consider using the X-UA-Compatible tag on your site? With the changes Microsoft has made to its browsers (more on those below).Depending upon what Microsoft browsers you support you may not need to continue using the X-UA-Compatible tag. If you need to support IE9 or IE8, then I would recommend using the tag. If you only support the latest browsers (IE11 and/or Edge) then I would consider dropping this tag altogether. If you use Twitter Bootstrap and need to eliminate validation warnings, this tag must appear in its specified order. Additional info below:The X-UA-Compatible meta tag allows web authors to choose what version of Internet Explorer the page should be rendered as.

IE11 has made changes to these modes; see the IE11 note below., the browser that replaced IE11, only honors the X-UA-Compatible meta tag in certain circumstances. See the Microsoft Edge note below.According to Microsoft, when using the X-UA-Compatible tag, it should be as high as possible in your document head:If you are using the X-UA-Compatible META tag you want to place it as close to the top of the page's HEAD as possible.

Internet Explorer begins interpreting markup using the latest version. When Internet Explorer encounters the X-UA-Compatible META tag it starts over using the designated version's engine. This is a performance hit because the browser must stop and restart analyzing the content.Here are your options:. 'IE=edge'.

'IE=11'. 'IE=EmulateIE11'. 'IE=10'. 'IE=EmulateIE10'. 'IE=9'. 'IE=EmulateIE9.

'IE=8'. 'IE=EmulateIE8'. 'IE=7'. 'IE=EmulateIE7'.

'IE=5'To attempt to understand what each means, here are definitions provided by Microsoft:Internet Explorer supports a number of document compatibility modes that enable different features and can affect the way content is displayed:.Edge mode tells Internet Explorer to display content in the highest mode available. With Internet Explorer 9, this is equivalent to IE9 mode. If a future release of Internet Explorer supported a higher compatibility mode, pages set to edge mode would appear in the highest mode supported by that version. The difference is that if you only specify the DOCTYPE, IE’s Compatibility View Settings take precedence. By default these settings force all intranet sites into Compatibility View regardless of DOCTYPE.

There’s also a checkbox to use Compatibility View for all websites, regardless of DOCTYPE.X-UA-Compatible overrides the Compatibility View Settings, so the page will render in standards mode regardless of the browser settings. @ClaraOnager Although Microsoft says it, it doesn't necessarily mean they're right. I've used since the day it came out without a problem at all. Actually, I've saved my team and myself hundreds of headaches already by making users' IE use its latest engine to render the pages we build. California interim driver license restrictions. Contrary to you and Microsoft, I do recommend everyone to use the above meta tag every time.

As long as IE is still around, we'll be 'forced' to use this meta tag:p–Jun 18 '13 at 21:56. This has changed with IE11. This version has taken a dramatic move to the world of browsers following official standards. It has gone to the extremes as to not even identifying itself as Internet Explorer! Now it says it's 'Netscape' and not including anything in the browser info to give away it's true identity.

If you still encounter any quirks in the IE browser from this version on, you need to force it into IE10 by setting. Then it reports itself as Microsoft Internet Explorer.–Nov 28 '13 at 7:47.