Send bitmap screen output directly to a file

Question

The question Set an HTML page as the wallpaper on Linux got me thinking... is there a way to open an application so that the raster output which would otherwise be displayed on the screen is rendered as a bitmap file instead?

Let's say (as is the case in the question above) that I want to render html as a bitmap every minute... both the Firefox rendering engine (Gecko) and WebKit are perfectly capable of handling this – but I want the output to go to a file instead of to the screen.

Is there a screen bitmap device that I could re-direct to a file, or a wrapper which would capture this?