This is important for web games. There are a lot of problems. System zoom and browser zoom interfere with how this is handled.
Problems I usually do not see for downloadable games. A downloadable might run internally on resolution A and just scales up or down to resolution B for the screen.
Actually using the native resolution, like a widescreen display, is harder. Is there no css like function in those game engines? Where you say, this ui element is supposed to be rendered at 10% screen height? Do you really spell out the pixel height and need to test this out for different resolutions? I would assume that game engines over the years would solve this and make this more plug and play for developers.