I looked with IE6, and the post reply and subscribe buttons take up the whole screen width. The reply itself is the exact same width, but shifted to the right due to the boxes there. I am also trying it in the IE 7 beta from Windows Vista, which does the same thing. When I get more time I will do Lynx and Links screens. I don't have cygwin installed which prevents me from getting screens of konqueror, which uses the same rendering engine as Safari.

Opera 8.5

Internet Explorer 6 SP1

Internet Explorer 7 for Windows Vista