I think you may be saying this sarcastically, but in response to the comment you replied to, at least my browser (Firefox) does do this. On this page, "ycombinator.com" is highlighted in white.
Chrome does not display any significant difference... or if it does, it's too subtle for my 20-40 vision.
Firefox is noticeable, now that you mention it. I never noticed before. A for effort, C- for execution?
Given the security implications, how about bold and blue? Maybe even a bigger font size? It should stand out, no?
(Are there any scripts that don't work at all with an underline? Bold not working for Chinese is a reason a bold domain isn't really an option.)