November 2021 Update
As this suffice is immediately 10+ years old my recommendation would be to leave this rag out raw, unless you must support old bequest browsers .
October 2015 Update
This answer was posted respective years ago and immediately the question very should be should you flush 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 wholly. If you use Twitter Bootstrap and need to eliminate establishment warnings, this tag must appear in its intend ordain. Additional information below :
The X-UA-Compatible meta tag allows web authors to choose what translation of Internet Explorer the page should be rendered as. IE11 has made changes to these modes ; see the IE11 notice below. Microsoft Edge, the browser that replaced IE11, entirely honors the X-UA-Compatible meta chase in sealed circumstances. See the Microsoft Edge notice below .
According to Microsoft, when using the X-UA-Compatible tag, it should be a 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 potential. Internet Explorer begins interpreting markup using the latest version. When Internet Explorer encounters the X-UA-Compatible META tag it starts over using the indicate interpretation ‘s locomotive. This is a performance shoot 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 entail, here are definitions provided by Microsoft :

Internet Explorer supports a issue of document compatibility modes that enable different features and can affect the manner subject 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. Those same pages would still appear in IE9 mode when viewed with Internet Explorer 9.
    Internet Explorer supports a number of document compatibility modes that enable different features and can affect the way content is displayed:
  • IE11 mode provides the highest support available for established and emerging industry standards, including the HTML5, CSS3 and others.
  • IE10 mode provides the highest support available for established and emerging industry standards, including the HTML5, CSS3 and others.
  • IE9 mode provides the highest support available for established and emerging industry standards, including the HTML5 (Working Draft), W3C Cascading Style Sheets Level 3 Specification (Working Draft), Scalable Vector Graphics (SVG) 1.0 Specification, and others. [Editor Note: IE 9 does not support CSS3 animations].
  • IE8 mode supports many established standards, including the W3C Cascading Style Sheets Level 2.1 Specification and the W3C Selectors API; it also provides limited support for the W3C Cascading Style Sheets Level 3 Specification (Working Draft) and other emerging standards.
  • IE7 mode renders content as if it were displayed in standards mode by Internet Explorer 7, whether or not the page contains a

    directive.

  • Emulate IE9 mode tells Internet Explorer to use the

    directive to determine how to render content. Standards mode directives are displayed in IE9 mode and quirks mode directives are displayed in IE5 mode. Unlike IE9 mode, Emulate IE9 mode respects the

    directive.

  • Emulate IE8 mode tells Internet Explorer to use the

    directive to determine how to render content. Standards mode directives are displayed in IE8 mode and quirks mode directives are displayed in IE5 mode. Unlike IE8 mode, Emulate IE8 mode respects the

    directive.

  • Emulate IE7 mode tells Internet Explorer to use the

    directive to determine how to render content. Standards mode directives are displayed in Internet Explorer 7 standards mode and quirks mode directives are displayed in IE5 mode. Unlike IE7 mode, Emulate IE7 mode respects the

    directive. For many web sites, this is the preferred compatibility mode.

  • IE5 mode renders content as if it were displayed in quirks mode by Internet Explorer 7, which is very similar to the way content was displayed in Microsoft Internet Explorer 5.

IE10 NOTE: As of IE10, quirks manner behaves differently than it did in earlier versions of the browser. In IE9 and earlier versions, quirks mode restricted the web page to the features supported by IE5.5. In IE10, quirks manner conforms to the differences specified in the HTML5 specification .
personally, I always choose the http-equiv="X-UA-Compatible" content="IE=edge" meta rag, as older versions have plenty of bugs, and I do not want IE to decide to go into “ Compatibility manner ” and show my web site as IE7 vs IE8 or 9. I constantly prefer the latest translation of IE .
IE11
From Microsoft :

Starting with IE11, edge mode is the prefer text file mode ; it represents the highest support for modern standards available to the browser .

Use the HTML5 document type announcement to enable edge modality :

Edge manner was introduced in Internet Explorer 8 and has been available in each subsequent secrete. note that the features supported by edge modality are limited to those supported by the specific version of the browser rendering the message .

Starting with IE11, document modes are deprecated and should no long be used, except on a impermanent basis. Make certain to update sites that rely on bequest features and document modes to reflect modern standards .

If you must target a specific document modality so that your site functions while you rework it to support modern standards and features, be mindful that you ‘re using a transitional feature of speech, one that may not be available in future versions .

If you presently use the x-ua-compatible heading to target a bequest document modality, it ‘s possible your locate wo n’t reflect the best experience available with IE11 .

Microsoft Edge (Replacement for Internet Explorer that comes bundled with Windows 10)
information on X-UA-Compatible meta tag for the “ Edge ” adaptation of IE. From Microsoft :

Introducing the “living” Edge document mode

As we announced in August 2013, we are deprecating document modes as of IE11. With our latest platform updates, the motivation for bequest document modes is primarily limited to Enterprise bequest web apps. With new architectural changes, these bequest document modes will be isolated from changes in the “ living ” Edge mode, which will help to guarantee a much higher level of compatibility for customers who depend on those modes and help us move even faster on improvements in Edge. IE will however honor document modes served by intranet sites, sites on the Compatibility View list, and when used with Enterprise Mode alone .

public Internet sites will be rendered with the modern Edge mode platform ( ignoring X-UA-Compatible ). It is our goal that Edge is the “ living ” document manner from here out and no promote document modes will be introduced going forward .

With the changes in Microsoft Edge to no longer support text file modes in most cases, Microsoft has a cock to scan your web site to check and see if it has code that is not compatible with Edge .
Chrome=1 Info for IE
There is besides chrome=1 that you can use or use together with one of the above options like : . chrome=1 is for Google ‘s Chrome Frame which is defined as :

Google Chrome Frame is an open reservoir browser circuit board. Users who have the circuit board installed have access to Google Chrome ‘s open network technologies and rapid JavaScript locomotive when they open pages in the browser .

Google Chrome Frame seamlessly enhances your browsing feel in Internet Explorer. It displays Google Chrome Frame enabled sites using Google Chrome ’ s rendering engineering, giving you access to the latest HTML5 features ampere well as Google Chrome ’ s performance and security features without in any means interrupting your common browser use .

When Google Chrome Frame is installed, the web just gets better without you having to think about it .

But for that circuit board to work you must use chrome=1 in the X-UA-Compatible meta tag .
More info on Chrome Frame can be found here .
Note: Google Chrome Frame merely works for IE6 through IE9, and was retired on February 25, 2014. More information can be found hera. Thanks to @ mck for the link .
Validation:
HTML5 :
The page will validate using the W3 Validator alone when using . For other values it will throw the error : A meta element with an http-equiv attribute whose value is X-UA-Compatible must have a content attribute with the value IE=edge. In other words, if you have IE=edge,chrome=1 it will not validate. I ignore this error completely as modern browsers plainly ignore this line of code .
If you must have completely valid code then consider doing this on the waiter level by setting HTTP header. As a note, Microsoft says, If both of these instructions are sent (meta and HTTP), the developer's preference (meta element) takes precedence over the web server setting (HTTP header). See olibre ‘s answer or bitinn ‘s answer for more details on how to set an HTTP header .
XHTML
There is n’t an topic with validation when using adenine long as the tag is by rights closed ( i.e. /> volt > ) .
Twitter Bootstrap (V3 and below)
This rag has been powerfully recommended by the Bootstrap team since at least 2014, and Bootlint, the linter authored by the twbs team continues to throw a warn when the tag is omitted. The linter distinguishes between warnings and errors, and as such the asperity of omitting this tag may be considered minor.

For more information on X-UA-Compatible see Microsoft ‘s Website Defining Document Compatibility .
For more information on what IE supports see caniuse.com .
For more information on Twitter Bootstrap requirements, see the bootlint project wiki page .

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