Firefox, Chrome, Safari, Internet Explorer, Opera. Developing websites to match all these browsers is no easy job. And don't even start talking about the different release versions. You can't force your visitor to use a specific software but your budget is limited and you can't create a whole new html for each one of them.
Luckily the modern versions of these browsers are relatively compatible, one difference here, another there. Nothing that a separate stylesheet or a reset css can't solve. One of them, however, take this to the extreme. Yes, I am talking about the evil IE6.
Losing Support?
IE6 is so full of bugs and rendering problems that several websites are not supporting it any more. Digg, Youtube, Orkut and all 37 signals products (Basecamp, Campfire...) just to name a few. Personally I don't design for IE6 unless the client specifically asks for it.
To Do or Not To Do
It is a tough decision to make, even tougher without the proper information. A good start is to analyse the average visitor of your (or your clients) website. Check their OS, browser software and work from there.
If a significant portion of the visitors have outdated browser then you might have no choice but to design for them. There are some information out there that can help you with the most common issues.
In case you have no traffic data you can check generic worldwide statistics. W3C, StatCounter and ADTECH (Europe) are a few places that display this information online. Be aware that these statistics represent a very wide audience that maybe don't represent your audience. In this website for instance, less then 1% of my visitors use IE6. Mainly because this website`s targets are developers, designers and general web-savvy people.
Happy End?
According to W3Schools, IE6 dropped 6% from January to September of this year. So lets be positive and one day we won't have to bother about this particular browser. I mean, even Microsoft launched a campaign to promote browser upgrade. Fingers crossed!