Drawing being done from the HTML itself would be too slow when user is scrolling the page [1]. That's why browsers use intermediate buffer/tiles into which the engine renders the page, and where the frontend blits it from.
But not on atari systems... even if the standard VDI wouldimplement offscreen buffers, I wouldn't use them for an application like netsurf. Because I don't want to allocate an buffer which is maybe 2500 x 60000 pixels large.