Best lightweight, responsive GUI platform Linux
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I've bought a Raspberry Pi, featuring a 300 Mhz CPU, but it does has a pretty good GPU. It can even run XBMC. I want to program a GUI for it, which needs to be responsive and good-looking, while taking optimal use of the hardware available (which isn't too good, unfortunately..). The Raspberry Pi runs Debian minimal, and I've put the X Windowing System on it, the rest is up to me to code.

What is the best way to dive into this? I've already looked into HTML5 and jQuery (via Python and PyWebkit), but they seem to be too CPU-intensive (unless someone knows how to get (Py)WebKit use the GPU, instead of the CPU).

Thanks in advance.