Allthough as Callum points out HTML5 and CSS3 is not wholly supported by IE as of yet, however it will not be long before the pop-ups (and inevitable TV ads) start appearing for IE9 in 2011 which is supposed to be a little more compliant with the new set of standards.
So web designers should expect the 'rank and file' joe public windows users to be switching over soon en mass. According to news from around the web Firefox dominates the European browser useage so updating some sites to use the basic new functions and learning about the new ones that are yet to be be completely cross browser compliant is probably a good idea.
Even Firefox does not completely support HTML5/CSS3. Dont get me wrong, i love firefox, and probably always will. The fact is that HTML5 and CSS3 are just being released (by "just" i mean in the past 6 months - year zone) and its gonna take a bit for browsers to support them completely.
As for w3schools, I have never run into any problems with w3, and their website is easy to navigate and to use a good resource for quick lookups. The site Alex Mac linked to is so bloated and blog-like, it would take forever to find a specific function.
W3Schools fails when it comes to best practices, and it doesn't teach from useful conceptual models.
W3Schools doesn't teach HTML 4 and XHTML semantically. Worse, they use presentational elements such as <b> and don't cover when it's OK to use <br/> (which is rarely, since <p>, <ol>, <ul>, <dl> and other elements will usually accomplish whatever effect the designer is going for while encapsulating more meaning). At one point, they may have taught table-based layouts well after CSS became widely supported in browsers, but I couldn't confirm it myself. They don't even cover structure vs. presentation, barely mentioning it in their "advanced" tutorial series. At least their last basic CSS tutorial mentions the problems of inline style, but it comes off as a minor point (if only they also mentioned this in there HTML tutorial when covering the style attribute). Their coverage of HTML5 may be better, but my suspicion is that's due more to presentational elements being removed from the standard.
Their JS tutorial uses document.write for dynamic HTML, and claims that assigning to an undeclared variable is the same as assigning to a declared one (a recipe for disaster).
Their PHP+MySQL tutorial is based on the twice-supplanted mysql driver. They often don't handle errors. Worst of all, they don't cover injection, SQL or otherwise. No mysqli nor PDO means no prepared statements.
These are only some of the problems I've come across when looking for material for others. There could easily be more problems awaiting discovery.
Search around, and you'll find the general opinion of intermediate and advanced web developers is that W3Schools reference sections are fine, but the tutorials are flawed, including: