I'd say it is possible, A List Apart has an example.

The example uses loads of javascript to show info when hovering, but just putting the "action points" (dt:s) on the image is done with plain css (the "showing info" part is possible to do in plain css as well, but it won't work in msie, which doesn't support the :hover selector).

I'm wondering how a mouse-free user makes use of these Norm? Is there any keyboard access at all?

Good point, Dave. I haven't investigated keyboard shortcuts yet. There is a provision for alternate text for each callout, though I've failed to use it in these examples :-( I'll try to fix that.

Excellent idea ! Unfortunately people like me are not very familiar to process perl scripts from java xslt engine through processing instructions. It would be great to put the same functionality in java into xalan and saxon extensions. Both of them support extensibility. Moreover this would make this process portable on every java supporting platform.