CODE
[root={param}]{content}[/root]
---becomes---CODE
<span style="position: relative; left: 0.8em; font-size: 60%; vertical-align: super;">{param}</span>√<span style="text-decoration: overline">{content}</span>
It's quite lovely.
I had some other ideas too, but I've forgetten them. I'll just add them if I remember.
-----Edit-----
& #8730; is the escape sequence for the root sign.
-----Edit-----
Unfortunately, it doesn't look too appealing when there's a superscript in the radicand.