Having a programmatic way of refreshing the desktop is what's required; it's already possible to change the wallpaper from a script, but the only way to make that take effect is by crashing the desktop; if you are an over-ambitious BT keyboard user (typing too many keys while it tries to connect), or use certain interacting versions of modest and/or statusbar clock (alarm-related troubles), that happens often enough it's not necessarily useless, but in an ideal world, the API used by the wallpaper changer would be exposed.

Then it's just a matter of a daemon setting the wallpaper as often as desired, rendering it from an html if desired.

This scheme is better than an auto-update interval because you can update images on other bases than fixed time intervals. Also, I feel that html rendering capability in there is major bloat.