evolt.org – Browser Archive contains various versions of all major and minor browsers that can be downloaded and installed for testing sites under many conditions.
Category: browsers
Tips and guidance for designing for all browsers
NYPL: Style Guide explains XHTML guidelines and benefits and offers CSS tips.
IE 6 and CSS compliance
HTML Tip: New Headaches with IE 6 on NetMechanic site explains how Internet Explorer 6 interprets your style sheets.