Software developers have been doing it for years, so its about time web-designers start doing the same, tell people to update, update, and update
Developers are dependent on the OS as their platform, and its usually hard to port their applications to other OSes. That's where web-designers are dependent on the Browser, and i guess we can call that their "platform", so deciding which platform to support is very important.
We should however keep in mind, that its far easier to update the browser, than it is to update the OS. Which is also the main reason why we shouldn't be so picky about telling our users to keep it Updated.
Brugbart recommends that you don't support any specific browser or version, and that you only apply hacks for the latest browser versions. Such hacks should be removed when a new version of the browser is released, to encourage your users to upgrade.
As we get more devices and browsers, obviously we wont be able to test our sites in all of them. It is the responsibility of the developers of those devices, and browsers, to support the standards. Web-designers should be able to design a page, without ever having to preview their work in a browser.
There are select cases, where a web-designer can say, "hay this screen is only x pixel wide, so the design should be like y", and we do have methods to do just that. Either by using a flexible design, or by using something like media queries.
Some fear that they might lose customers if they drop support for certain browsers. However, people will in reality be willing to update, if the site in question is important enough to them.
What about System Setups?
Some people, or even smaller companies, might be using an impractical system setup, which doesn't allow users to update their browser.
This cant really be our problem, chances are that people accessing your website from their work, had no business doing so in the first place. They should likely be working!
If visiting your site was a part of their work, then its their system administrators responsibility to make sure that their browser is supported by your site. Just like they would make sure that their current OS is supported by Photoshop's developers, or whatever software they are using.
What about Win9x Users?
It should be mentioned that even win9x users have a choice, they can switch to Opera, which still runs quite well on these older systems. It can even feel like they just got a new computer.
The benefits from not being forced to fix browser specific bugs, should be enough to get any designer going. Sadly some web designers fear that they lose visitors by doing it, and that might be true in a few cases. But the majority of users will be willing to update their browser, given that they like the site enough. This nasty old habbit, of sticking with old browser versions, will also even out eventually.
PNG Alpha Transparency
Old 256 color Gifs and PNGs with index Transparency, should be well known to most web-designers about now. But we also have whats called PNG24 and PNG32, which are 24bit and 32bit respectively. These formats are both 24bit, but the 32bit one also has support for Alpha Transparency, which allows for richer design. That alone would be a pretty good reason for a lot of designers.
The File-size of these png-images, is also generally lower then that of normal Gifs. In addition, you can make use of CSS Sprites, to lower the file-size even more, and save http requests at the same time.
Data-URIs is another important benefit, which allows to lower http requests by quite a lot. Using them can allow you to keep images embedded directly in your external stylesheets, which means that they will be cached together with the CSS, and lower the http requests made to your server.