styling text

Hi,
What I'm trying to do is bound to be dead straightforward, but I can't for
the life of me see it in the spec.

All I want to do is define e.g. h1 as having style="font-size:100", h2 with
style="font-size:200" then call on these styles later (in the same doc),
like css. Something like :

<defs>
	h1  style="font-size:200"
	h2  style="font-size:20"
</defs>

...

<blah>
<text style="url(#h1)">Big Headline</text>
<text style="url(#h2)">Smaller Headline</text>
</blah>

defs? It doesn't look like it...
directly as css tags? maybe...class...hmm...

It's late at night & I'm off to bed now, but I'd appreciate someone making
me feel stupid in the morning.

Cheers,
Danny.