HTML and CSS for sure (XHTML can't be properly handled by most server/browser combinations, although they fake it well). For front-end interactions, Javascript

HTML and CSS for sure (XHTML can't be properly handled by most server/browser combinations, although they fake it well). For front-end interactions, Javascript is pretty much the only answer. jQuery is just a Javascript library, and AJAX is just a method to send/receive information without refreshing the entire page. For "true" Web 2.0 functionality, you'll also need to know a server-side language. PHP is certainly one of the more popular ones (and among the easiest to get working), but there is also Java, Ruby, ColdFusion, .net (C#, ASP.net, VB.net,etc.), Perl, Python, LISP, ARC, etc. Chances are you'll need to be able to store information, which pretty much requires a database. MySQL is easy to work with, but there's also PostgreSQL, SQL Server and Oracle (among others). Personally, I currently use HTML 5, CSS, Javascript (with jQuery), PHP and MySQL most frequently. As a designer, however, your biggest concern is almost certainly going to be the HTML/CSS side of things. A library like jQuery will handle 99% of your cross-browser Javascript issues, but getting a page to look right (though not necessarily identical) across browsers can be a challenge. You'll need to know not just the markup (HTML) and display (CSS) languages, but also how each of those are interpreted in the major browsers (IE 7+, Firefox 3+, Chrome 4+, Safari 3+ and Opera 9+). You'll need to know how to make your PNG graphics look the same in Windows and OSX (gamma differences, if I recall correctly). Fortunately, all this information (and more, so much more) is at your fingertips. All you need is patience, time, and more patience :) Good luck!