HTML: What is `xmlns:fb=“http://www.facebook.com/2008/fbml”`?

Question

What is xmlns:fb=\"http://www.facebook.com/2008/fbml\"?

I\'ve been seeing it in a lot of tags recently. What does it do?

Answer 1

Just as @Matthew Scharley said, its used to embed Facebook-specific tags on your site. For example, here's our company's Facebook stream on our site. Another common use it the "Like" button on website. More information can be found on the Facebook developer site.

UPDATE

Here's some additional widgets/plugins.

Answer 2

While I haven't seen it in use, this is standard XML notation for extending the current XML namespace (in this case XHTML) with another one (in this case FBML). This tells your browser where to find the specification for FBML which in turn tells it how to handle the <fb:*> tags. Technically speaking, the value of the xmlns:* attribute is simply a unique identifier, but often it is a URL which points to a definition in one form or another. In this particular case the URL doesn't point to a definition unfortunately.

See W3Schools for more information about XML namespaces.