Did you have to get a licence to display this? TfL are pretty strict about reproducing their map, even more so if it's going to be available offline on a mobile device.
Naturally this CSS map is an almost 1-for-1 copy of the original. But how far would it have to diverge to not be copyright infringement? Different colours? Use rounder angles ?
There are also restrictions over the font, logos and symbols.
But where does the copyright infringement start? Could you technically get around it if you changed a couple of the colours and fonts around and used slightly different angles?
It might result in a few confused travellers, but hey.