The site is a fantastic collection of 37 articles and counting that lead from a general web introduction through HTML, accessibility and CSS. There are great articles on colour theory and web typography. The site is aimed at students and educators in the hope they will teach their students the right way to do things on the web.
The W3C may publish the standards but the Opera Web Standards Curriculum is a great practical resource to learn them.
When I do web development, my browser of choice is Firefox. With geat extensions like Web Developer, tracking down HTML / CSS issues is made so much easier. There are plenty of others like Live HTTP Headers and Search Status to name just 2.
However when I am just browsing the web, my browser of choice is Opera, and has been for a while now. Performance wise I have found it better than firefox. This is just a perception, without any rigorous analytics. part of the answer is probably to do with FF being slowed down by the installed extensions. Remember I am running Linux (Ubuntu), so Safari and Google Chrome are not available at this time. Konqueror is a good alternative when I was on KDE.
Yes Opera is not free software, just freeware, but for me in this case, its the best tool for the job.
A major challenge of web development is that a particular OS and browser of the end user is not guaranteed, and so many must be catered for. This is not as bad as it used to be, now that Netscape 4, and Internet Explorer 5 and earlier have been consigned to history. This leave IE6, 7 and soon 8, Firefox, Safari and Opera. That is still plenty of scope for variation, and the general problem is IE6. So the decision by 37Signals to discontinue support for IE6 in mid August is an interesting development. It is also a bold move, but that is the way 37 Signals so things, by not catering to the entire world.