Not applicable to most games, but I did a puzzle game that uses HTML/CSS for layout, then enforced a fixed 2:3 ratio for the "content" area of the game, with ea

Not applicable to most games, but I did a puzzle game that uses HTML/CSS for layout, then enforced a fixed 2:3 ratio for the "content" area of the game, with each UI element using a percentage-based width/height. The great thing about this method is that the "graphics" are always crisp, regardless of screen resolution or pixel density.