How hard would it be to make a fullscreen demo?

I want to teach myself to make games in HTML5 and most of the examples I see use fixed size frame; so I must be fair to assume going fullscreen is a hard problem.

The games I'm interested are visual novels http://en.wikipedia.org/wiki/Visual_novel or Undum-based interactive fiction http://almostgoodbye.textories.com/ upgraded with full-screen static 2D graphics and music, so graphics perfomance requirements are quite modest, but running fluidly on screens of different resolution is a must.