UPDATED: After some Twitter conversation with @Phil_Wheeler, @brianblakely and the @h5bp team, Brian has added info about the X-UA-Compatible meta tag go the HTML5 Boilerplate Head Options page. If you are not familiar with the HTML5 Boilerplate project, you should definitely check it out.
The <meta> tag is one of the most commonly used tags within the <head> element of many web pages. <meta> tags are used to convey all types of metadata about a given page to the browser or a search engine spider, including:
The Internet Explorer team recently announced that the “metro” version of IE10 would begin to support a new <meta> tag for sites that require the use of browser plug-in’s like Adobe Flash and Microsoft Silverlight:
<meta http-equiv="X-UA-Compatible" content="requiresActiveX=true" />
The X-UA-Compatible <meta> tag forces IE10 to pop up a dialog asking the user if they would like to open the current page in the “desktop” version of IE10, which does support browser plug-ins. The dialog looks like this:
There are so many useful <meta> tags that it’s hard to keep track of their name’s, values and purposes. Luckily, there is a pretty good resource to help out with that:
Brian Blakely has a great Gist that he calls “Helpful things to keep in your <head/>” which does a good job of documenting many of the most popular <meta> tags. I’ve forked his Gist and added the X-UA-Compatible tag. Unfortunately, GitHub doesn’t allow for pull requests on Gist’s, so I’ve asked Brian to include X-UA-Compatible in his Gist via a comment.
I think that it would be helpful to turn Brian’s Gist into a full fledged <meta> tag database, which could also include useful <link> tags, such as those used for OpenID delegation. I’ll add the idea to my personal backlog – unless you, dear reader, want to take that on.