Those of you hoping to make use of highly advanced CSS should make sure that your readership are clued-in to the technology.
Coding a site about antiques or gardening using cutting-edge techniques is not always going to be a good idea. It shouldn't be too hard to avoid using these tags completely.
While at the end of the 90s the trend was to use hacks and lots of presentational markup to have your site looking comparable across browser versions, this practice is not advisable anymore.
Use CSS to pretty-up your pages for readers with capable browsers, but don't fill your code with lots of extraneous tags just to service the users of old browsers (while dumping a large filesize and compatibility penalty on those with a modern browser).
Check out our Browser Support page and chart for a generalised look at which browsers support what.
Your decision on what level of the technology you want to use will have to be made up depending on your requirements and audience.
Don't use the old 'best experienced in browser X' cliché, you'll be driving visitors away for no reason.
A well designed site has no need for weak excuses like this.
It's ok to have your pages looking bare in old browsers — making sure they're accessible should be a much higher concern.
As mentioned already, you should have a good knowledge of which tags came with which HTML recommendation.
to test every single possible technical configuration, there are things that must work in broad areas, and things you can go around.
Page Navigation: Graceful Degradation | Browser Compatibility | Messed-up code | Colour Depth | Resolution | Images This page was last updated on 2012-08-21 While the current generation of browsers have indeed all done an excellent job of supporting established » web standards, you must remain aware that there will always be a proportion of your website's audience using browsers that don't support the latest hot web technologies, such as Ajax. While it would be an ideal situation if everybody diligently downloaded the newest browser on each release, it is simply not acceptable to expect this.
Of course, if you want the maximum and most diverse range of readers, aim as low as possible.