If you REALLY value compactness, the smallest nice computer you can get is probably the discontinued 12-inch macbook. It really is amazingly small. It is underpowered but "quickly write tiny JS/PHP/Html programs" is about the least demanding thing you can do, work wise, so I think it would be fine.

It's a sacrifice for compactness though. It also has the "bad" keyboard, though some people don't mind.

The just released macbook air is probably an all around good choice though.