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.

If you're looking to make it technically valid (everyone loves to see the green favicon) w/o effecting any functionality, you should be able to just wrap it in a "if IE" tag.

<!--[if IE]><meta http-equiv='X-UA-Compatible' content='IE=edge,chrome=1'><![endif]-->

One possible solution is to implement a fix server-side in the header, as suggested in this nice write-up by Aaron Layton. (All credit should go to him, and I'll paraphrase rather than plagiarize...)

<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1" />

"When Internet Explorer comes across this line it will change the engine that is being used to first Chrome Frame, if the plugin is installed, and then to Edge (the highest supported document mode of the browser)."

Steps:

• Fix the page validation – This is achieved by simply removing the tag
• Rendering speed – Instead of waiting for the browser to see the tag and then change modes we will send the correct mode upfront as a response header
• Make sure that we only show the fix for Internet Explorer – We will just use some server side browser detection and only send it to IE

To add the header in PHP we can just add this to our page:

if (isset($_SERVER['HTTP_USER_AGENT']) &&
    (strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE') !== false))
        header('X-UA-Compatible: IE=edge,chrome=1');

Or you could add it to your .htaccess file like so:

<FilesMatch "\.(htm|html|php)$">
    <IfModule mod_headers.c>
        BrowserMatch MSIE ie
        Header set X-UA-Compatible "IE=Edge,chrome=1" env=ie
    </IfModule>
</FilesMatch>

Link to original article, check comments for possible caveats. Also includes an implementation for C#.

Fix Bad value X-UA-Compatible once and for all

Hope this helps!