Created attachment 288094 [details] test case Bold words in links, or any span with text-decoration:underline, have a double underline. This only happens when the Arial font is used. This only happens in quirks mode.
Yes, I see this, too, on Linux and OS/2. In the testcase of attachment 288094 [details] both the "Arail" and "Lucida Grande" cases show it, at least when the font size is large enough (using Ctrl-+), the "Times" case doesn't. In your screenshot I don't actually see a double underline, just an underline that is bold like the text. The underline thickness is actually a property of the font, and a bold font may well have a different value than a non-bold one (or another bold font). But that doesn't explain why it only happens in quirks mode.
(In reply to comment #2) > both the "Arail" and "Lucida Grande" cases show it, at least when the font size You almost certainly don't have the 'Lucida Grande' font installed, as it is OS X only. The test case then defaults to Arail (which should be 'Arial', can't seem to spell that one right..). And, yeah, the underline thickness is dependent on the font (and font-weight), but in this case, the whole text-run should have the same underline thickness (per the specs), as it does for most, if not all, fonts I have installed (didn't test them all). Thanks for confirming the issue on a non-mac platform. That makes this bug probably dependent on bug 402524. (and I added 'text-decoration' in the summary).
This should be marked as WONTFIX. We don't prefer the text-decoration spec in CSS at quirks mode. The cause is that nsTextFrame draws the underlines per the frames.
Landing bug 403524 merged the standards-mode and quirks-mode text-decoration code, so quirks-mode now complies with that portion of the CSS2.1 spec