Doesn't always work (may work for your use case). There are edgecases in the browser where at an HTML level the browser computes a CSS size but at compositing l

Doesn't always work (may work for your use case). There are edgecases in the browser where at an HTML level the browser computes a CSS size but at compositing level (where devicePixelRatio is used) it might have clip by pixel here or there. And example, lets say your windows 99 device pixel wide and a DRP of 2.0 and lets say you ask for 2 side by side elements (via flexbox or whatever) each width:50%. At the HTML level the browser will compute there are 49.5 CSS pixels available. It will then compute the size of each 50% as 24.25 CSS pixels but in order to fill the 99 pixels, one of those 2 elements has to be 50 pixels and the other 49. There is no part of the spec that says which side gets 50 and which get 49. Most browser compute 50 for both sides, the pass it up their compositor which will use 50 pixels of one and 49 of the other. The point is, you can't know which one will get which by doing CSSpx * dpr. This is why ResizeObserver device-pixel-content-box was added. So that you can ask the browser what it did.