What is the syntax for the HTML5 media attribute value with an expression?

I was wonderin开发者_如何学Pythong what is the syntax for the media attributes value with an expression?

The standard for HTML5 says:

The media attribute gives the intended media type of the media resource, to help the user agent determine if this media resource is useful to the user before fetching it. Its value must be a valid media query.

Follow that second link for the full syntax details.