Brunelleschi 1.5.5 causes IE 8 (on Windows XP) to crash before the page finishes loading. I don't have a Windows machine, but this is from a reliable source with a brand new computer.
As of this writing, IE 8 is the most widely used version of that browser, with almost double the users of IE9, according to browser stats at w3schools.com. I realize IE is probably the most despised browser among developers, but I think the user base is too large to ignore.
I tried 1.5.6 from the author's web site (it's not at the WP site yet); it doesn't cause the crash, but it causes major formatting problems in the latest versions of Safari, Firefox, Chrome, and Opera on my Mac (Mac Pro, 16 GB RAM, OS 10.6.8) as well as in Firefox 10 and IE 8 on Windows XP.