My recollection is that type="..." is advisory: it helps user agents
optimize for the case that they don't know the relevant media type,
so they can skip fetching the thing. So it would be odd for it
to be mandatory. But sure enough! it is:
I wonder why it's mandatory.
Because typically, CSS processors cannot deal with XSL stylesheets and
XSL processors cannot deal with CSS stylesheets, and avioding
downloading the thing if it is not a type you can process is highly
desirable.
This reasoning doesn't apply when the stylesheet is embedded, which
is the case we're complaining about. It's fine to supply a type
for an external stylesheet, it isn't fine to supply (or require
supplying) a type for embedded stylesheets that are referenced
via a fragment identifier.
Larry