If the specified node has a `media' attribute, then this
routine extracts and parses it, according to the scheme described in
the HTML4 spec at
<http://www.w3.org/TR/REC-html40/types.html#h-6.13>. That is,
media types are separated by commas plus whitespace, and each media
type may have an arbitrary parameter string separated from it by
whitespace.
The node to be examined for a media attribute.
Type: list of pairs
A list of pairs, with the car of
each one the media type, and the cdr any parameterstring present. If
there is no `media' attribute, return #f.