New in HTML5.

<source> HTML Tag

Element of
HTML Media
What does <source> HTML Tag do?
The <source> element is used as a child of a <picture>, <audio>, or <video> element, and identifies the URL of one or more media resources. The <source> element is commonly used to add media resources in multiple formats for the best possible cross-browser compatibility.
Null element
This element must not contain any content, and does not need a closing tag.

Alternate Sources

The purpose of <source> is to define multiple versions of a file, so you can specify different versions for different browsers or platforms. You can use <source> within either <audio>, <track> or <video> elements.

Browser Support for source