OK, I understand your answer now, thanks. But I didn't get closer to what I want to achieve. Now that I understand the subject a bit more I should ask: is there a way to format the tooltip help for elements and attributes (values) in Oxygen? I use the Relax NG XML grammar and Oxygen 11.
In particular, it would be lovely if we could associate some CSS with the annotations in a RELAX NG grammar. (After all, the tooltip is just a box -- I realize it might be just a box of text, but it may be a box that has gone through a rendering engine for all I know.) I, of course, use the TEI elements designed for annotating XML constructs. E.g. <tei:desc>The <tei:att>algorithm</tei:att> attribute need not be specified on the <tei:gi>sort</tei:gi> element, but if it is not specified, the application picks the algorithm at random. Woe to you if it picks <tei:val>blort</tei:val>!</tei:desc> And would love to be able to render those in a pretty way. (And if oXygen can already do this, just tell me how, and smirk to yourselves as I congratulate you and a job already well done. :-)