The area of the browser window a page can actually use, in CSS pixels. Resize the window and watch it update live. The breakpoint follows Tailwind's defaults (sm 640 · md 768 · lg 1024 · xl 1280 · 2xl 1536); Bootstrap's are similar.
The proportion of width to height.
Physical pixels per CSS pixel. Browser zoom changes this value too.
The physical pixels of your display panel: screen size multiplied by the device pixel ratio.
Bits your screen uses to describe the color of a single pixel.
Whether your screen is currently wider than it is tall.
Screen space minus the OS taskbar, dock, and menu bar — what windows can actually occupy.
The range of colors your display can show. P3 and Rec. 2020 are wider than standard sRGB.
Estimated from animation-frame timing, so browser throttling or heavy load can lower it. Your panel's spec is the ceiling.