Defines a composition that optionally uses a template, as outlined in the description
of the ui tag library. Multiple compositions can use the same template, thus encapsulating
and reusing layout. JSF disregards everything outside of the composition, which lets
developers embed compositions in well-formed XHTML pages that can be viewed in an XHTML viewer,
such as Dreamweaver or a browser, without including extraneous elements such as head and body.
A URI that points to a template, also known as a layout, that inserts pieces of the page defined in the composition.