This allows both boolean and string-type preferences. It
also implements a system where prefs that are read from a
configuration file can be reset back to their initial value,
which is useful in a number of cases e.g. when running tests
to ensure that each test starts with the same values for
the prefs.
This is for parsing of the rule within a stylesheet only. <meta name=viewport> could create a viewport rule and add it to the list of stylesheets for the page (like quirks mode).