Bad Value X-Ua-Compatible for Attribute Http-Equiv on Element Meta

Bad value X-UA-Compatible for attribute http-equiv on element meta

Either X-UA-Compatible is not "standard" HTML (FSVO "standard" that involves appearing on a publicly editable wiki page referenced by the specification) or the Validator isn't up to date with the current status of that wiki.

At the time of writing (20130326) X-UA-Compatible appears on the wiki page under a section that states: "The following proposed extensions do not yet conform to all the registration requirements in the HTML spec and are therefore not yet allowed in valid documents." So the validator is correct to reject this value.

Bad value X-UA-Compatible

You're getting this because X-UA-Compatible is not a valid value for http-equiv on the meta tag, according to the W3C Validator.

The X- prefix denotes a vendor specific custom extension, just like a data- attribute. So, in summary, although this makes sense, the standard dictates it is wrong, so the validator itself is correct. You can read the full W3 spec, which contains a list of the expected values for http-equiv.

As for where this is coming from, it's likely in your header.php file of your theme, probably the first line.

The woo_head() method itself is part of WooThemes, and their framework.

Error: Bad value X-UA-Compatible for attribute http-equiv on element meta

On Apache, add this to your root .htaccess file:

Header set X-UA-Compatible "IE=edge"

Check this article for more info on the error: http://www.456bereastreet.com/archive/201103/x-ua-compatible_and_html5/

Bad value expires for attribute http-equiv on element meta

You are not allowed to use the expires pragma directive in HTML5, as it is not defined in the HTML5 spec nor registered on the wiki page PragmaExtensions.

Instead, you should use the corresponding HTTP header.

According to RFC 4229, the Expires header field is defined in RFC 2616, and its definition is pretty clear about the format:

The format is an absolute date and time as defined by HTTP-date in section 3.3.1; it MUST be in RFC 1123 date format […]

Note that it also says about the 0 value:

HTTP/1.1 clients and caches MUST treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired").

About what you should do:

To mark a response as "already expired," an origin server sends an Expires date that is equal to the Date header value.

Bad value cache-control for attribute http-equiv on element meta

HTML5 has very small set of values available for http-equiv attribute for meta property. It's not that you can take any possible HTTP header and use it this way. Although browser may and support many non-standard values, it's simply non spec-conformant.

Information here: http://www.w3.org/TR/html5/document-metadata.html#standard-metadata-names in p. 4.2.5.3 Pragma directives.

What you can probably do is - depending on used server technology - add those headers to HTTP response. How - this depends on whether your page is dynamically generated or is a static file. In the second case it depends on server software capabilities.

What does meta http-equiv=X-UA-Compatible content=IE=edge do?

November 2021 Update

As this answer is now 10+ years old my recommendation would be to leave this tag out altogether, unless you must support old legacy browsers.

October 2015 Update

This 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:

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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. Microsoft Edge, 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. 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 <!DOCTYPE> directive.

• Emulate IE9 mode tells Internet Explorer to use the <!DOCTYPE> 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 <!DOCTYPE> directive.

• Emulate IE8 mode tells Internet Explorer to use the <!DOCTYPE> 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 <!DOCTYPE> directive.

• Emulate IE7 mode tells Internet Explorer to use the <!DOCTYPE> 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 <!DOCTYPE> 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 mode behaves differently than it did in earlier versions of the browser. In IE9 and earlier versions, quirks mode restricted the webpage to the features supported by IE5.5. In IE10, quirks mode conforms to the differences specified in the HTML5 specification.

Personally, I always choose the http-equiv="X-UA-Compatible" content="IE=edge" meta tag, as older versions have plenty of bugs, and I do not want IE to decide to go into "Compatibility mode" and show my site as IE7 vs IE8 or 9. I always prefer the latest version of IE.

IE11

From Microsoft:

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

Use the HTML5 document type declaration to enable edge mode:

<!doctype html>

Edge mode was introduced in Internet Explorer 8 and has been available in each subsequent release. Note that the features supported by edge mode are limited to those supported by the specific version of the browser rendering the content.

Starting with IE11, document modes are deprecated and should no longer be used, except on a temporary basis. Make sure to update sites that rely on legacy features and document modes to reflect modern standards.

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

If you currently use the x-ua-compatible header to target a legacy document mode, it's possible your site won'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" version 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 need for legacy document modes is primarily limited to Enterprise legacy web apps. With new architectural changes, these legacy 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 still honor document modes served by intranet sites, sites on the Compatibility View list, and when used with Enterprise Mode only.

Public Internet sites will be rendered with the new Edge mode platform (ignoring X-UA-Compatible). It is our goal that Edge is the "living" document mode from here out and no further document modes will be introduced going forward.

With the changes in Microsoft Edge to no longer support document modes in most cases, Microsoft has a tool to scan your site to check and see if it has code that is not compatible with Edge.

Chrome=1 Info for IE

There is also chrome=1 that you can use or use together with one of the above options like: <meta http-equiv="X-UA-Compatible" content="IE=Edge,chrome=1">. chrome=1 is for Google's Chrome Frame which is defined as:

Google Chrome Frame is an open source browser plug-in. Users who have the plug-in installed have access to Google Chrome's open web technologies and speedy JavaScript engine when they open pages in the browser.

Google Chrome Frame seamlessly enhances your browsing experience in Internet Explorer. It displays Google Chrome Frame enabled sites using Google Chrome’s rendering technology, giving you access to the latest HTML5 features as well as Google Chrome’s performance and security features without in any way interrupting your usual browser usage.

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

But for that plug-in 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 only works for IE6 through IE9, and was retired on February 25, 2014. More info can be found here. Thanks to @mck for the link.

Validation:

HTML5:

The page will validate using the W3 Validator only when using <meta http-equiv="X-UA-Compatible" content="IE=Edge">. 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 simply ignore this line of code.

If you must have completely valid code then consider doing this on the server 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 isn't an issue with validation when using <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> as long as the tag is properly closed (i.e. /> vs >).

Twitter Bootstrap (V3 and below)

This tag has been strongly recommended by the Bootstrap team since at least 2014, and Bootlint, the linter authored by the twbs team continues to throw a warning when the tag is omitted. The linter distinguishes between warnings and errors, and as such the severity of omitting this tag may be considered minor.

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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.

How to fix meta tag validation error thrown by W3C validator?

Assuming that you are validating an HTML5 page, you will have seen this warning at the top of the page:

Using experimental feature: HTML5 Conformance Checker.

The validator checked your document with an experimental feature: HTML5 Conformance Checker. This feature has been made available for your convenience, but be aware that it may be unreliable, or not perfectly up to date with the latest development of some cutting-edge technologies.

There are two issues that are causing your page not to validate: firstly, as the warning says, the html5 validator is only experimental and does not yet cover all features of HTML5, and secondly, the HTML5 specification itself is currently under development, and new versions are being proposed on a fairly frequent basis. Until the specification is confirmed, and the w3c validator implements checks for the meta tag attributes that you're using, you will continue to get errors.

Validator.nu is more up to date with recent candidate HTML5 specs if you want to check HTML5 documents.

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