head Element(Cont'd)

switch

The switch element specifies a set of alternative elements from which only one should be chosen. The player evaluates the elements in the order that they occur in the switch. The first element for which the media-type can be decoded and all of the test-attributes evaluate to true is selected, and all other elements in the switch element are excluded.
The switch element has two attributes: "id" and "title." The id attribute simply gives a unique identifier to the element. The title attribute gives advisory information about the element.
When used in the head element, switch can only have children of type layout.
example