Redmine

E.g., when one writes a comment, in the activity view the line with the description (which is in <span class="description"> tags) should be rendered somehow and not just shown as plain text.

It doesn't need to be or it sould not be textile/markdown rendering, since I wouldn't expect headings or clickable links. Just strip off any formatting, show only the text of links, remove images, ... I don't know what else has to be done.