Web export: Ability to set it up so it won't be stretched.

Stretching to window/device width/height is cool and all, but sometimes you don’t want it, e.g. when you want to display some text along the game (and iframes are ugly and smell of elderberries). So my suggestion is to add setting that would enforce absolute dimensions of a game in pixels that are set in project settings (game would be displayed in the tope-left corner then and it’s up to the user to set it up in center if he/she wants it).

Tried to do this with CSS hack but as it turns out GDjs sets style directly in the canvas element which makes any size adjustments impossible.

Did you try to disable fullscreen using the fullscreen action? By default HTML5 games are set as being “fullscreen” and taking as much space as possible on the webpage (while keeping their aspect ratio)