In browsers, there are supposed to be roughly 72px per inch. This should get scaled, assuming the OS/Browser knows what the actual pixel density per inch is. And even then the scaling may not be linear to reality.

If you've ever run a mobile device without the real dimensions set properly (cheaper models, or custom roms) it gets interesting to say the least. Having less than stellar vision up close, I have every accessibility setting on my phone maxed... It's bad enough as it is the number of applications/sites that will render (in particular modals) off-screen.

In the end, however, nearly everything is scaled to either the real pixels, or often assume 72/inch as the actual screen density and scale accordingly. You really never know which.