«In the front end, all links are rendered correctly, but in the RTE itself the links are shown in standard color (blue) with underline, except links I've already visited. These are in black with underline and a hover effect. If I add new links, it's the same: Visited links become black, unvisited blue. The rest of the style sheet is parsed without problems. Any hints?»

This is a Mozilla/Firefox feature. In the editing area, the link attributes defined in your profile take precedence over the corresponding attributes specified in your style sheet. Apparently, these preferences cannot be neutral.

In a editing area displayed by Internet Explorer, the style sheet specification is applied to the link.