(an answer to "Which browser to design for?")
Trick question. Answer: design for your audience and, to insure widest accessibility, make the code HTML4.0 (or HTML3.2) compliant.
This answer is not intuitively obvious when taking a Sunday stroll around the Web. Visiting too many web sites reminds me of early desktop publishing -- when just because it was possible to put 14 typefaces on a page -- some folks did. (Yes, I was around back then. I have the dubious honor of having used (Aldus) Pagemaker since the beginning ... as I made the transition from x-acto knives, wax and T-squares to digital layout and imaging.)
The Web does promise multi-media -- sound, words, pictures, interactivity. And for those with direct Net access ('big pipe,' TI connection, etc.), the promise can be delivered (sometimes). Average users, however, use a modem to dial up to the Net; huge files often test user patience, even in the age of 56K modems. DSL and cable reach only about 5 million American netizens.
Many web designers push the edge of the technology envelope, using frames, animations, flash, and a host of other plug-ins that may or may not be available for all browsers. Many "bend" HTML, a language that was designed to mark-up information, not to be the Pagemaker of the online world.
The question to ask before adding "extras" or using non-compliant code: Does this technology aid communication or is it the use of technology for technology's sake? Not all designers leap at the chance to load up a page with gizmos. A summary of a discussion of "pet peeves" among women who design web sites may surprise you. Especially since it's years later, and similar conversations continue to take place among designers.
Many sites cut off half (or more) of their potential visitors -- Netscape, old AOL, Lynx, braille-or-speech output. For a government site this is more than a shame -- it's cutting off access for a large portion of the citizenry who access the Net via public libraries, universities and other text-only vehicles. Some say this is an ADA (Americans With Disabilities Act) violation. Why wait for a court injunction when designing for accessibility is so straightforward?
Having analyzed Web sites since 1995, I have become a confirmed user advocate. Check out TFM, where I expound on interface errors (among other things!).
If Accessibility Is Your Goal