SlashGame has been gone for nearly a year. It's making a comeback, but it's been left in a mess.
There are a lot of things not working on the site at the moment. However, the most useful part of it is still here. Check out the HTML5 Game Development Tutorial for a single page teaching you a lot of the basics of HTML5 game development.
Or if you'd prefer this kind of layout, check out the HTML5 Game Development Guide, which is a bit more extensive and is broken up into different bits.