Short answer: All of them. We talked about this a little bit when discussing Shockwave in December. It's a fairly major issue so we'll go into a little more detail.
In an ideal world developers wouldn't have to worry about which browsers to design for. They would all follow some simple standards and support the same features. But because of the way the Internet industries are shaping up, everyone's in a rush to define the standards themselves, and reinvent them every few months with a new browser release. Therefore, Netscape Navigator 3.0 supports features that 2.0 doesn't, and Microsoft Internet Explorer supports a whole host of other tags as well.
The best answer is to get to know the differences between browsers. Be aware that some will show some of those features (say, font colors) and others will show other features, and think about the different combinations.
The good news is that browsers tend to ignore features and tags they don't understand. It's possible then to layer on tags for both Netscape features and Explore features. Each browser will see what you want them to and ignore the other tags.
I run a business-to-business Web site. How often should I update my pages?
This ties in well with another frustrated marketer who wrote IM and asked "Why don't more site designers use research to understand their Web market and audience before developing a site?"
It's not the designer's fault, really, except that the designer should be savvy enough to guide potential clients through the process. When a marketer is thinking about going on the Web, they had better be able to answer the big question: "Why?"
I have no idea why marketers go on the Web blindly, but they seem to do it often. It's especially inexcusable because you can generally ask for a lot of this kind of information right online. People will fill out surveys online, especially if you don't require their names. It's a quick and easy way to build up a database.
Regardless, here are some things to think about as you head for the Web, or perhaps redesign:
The answers to these questions will help you answer the "how often should I update" question. If your site is going to be set up to draw visitors back everyday and you have the staff to support it, you should have some new content up for them.
If you want people to look to your site when they want information about your product/service, then have a lot of detailed information about that. That kind of content needs to be current, but not necessarily refreshed daily.
Two more tips: Changes to the front page can save more time than changing all the pages underneath it.
Lastly, if you're not going to update your content a lot, do not put a "last updated" date on there. There's no need and it could make your site look neglected.