I should have been more clear with the centering issue: I'm looking at something like GSS [0] as a model of what would be incredibly useful.

As for IDEs, I've always used a text editor for web dev, but there's something incredibly fun about being able to draw components and move them around on your screen as opposed to having to mess around with something like the Canvas API. I built fairly complicated little apps with Flash when I was 10 that would take me 10x longer (at least) to implement in HTML5.

0: http://gridstylesheets.org/