Screen checker

A dead pixel no longer receives power, so it stays black whatever is on screen. It shows up most clearly against white.

A stuck pixel still has power but is fixed at one value, usually because a red, green or blue subpixel refuses to change. That shows up against black, or when you cycle the three primaries - the dot is only wrong on one of them.

Sometimes. The usual trick is to flash colours at that area for a few tens of minutes to "force" it back into action - switch the auto-cycle on at the 0.5 second speed and leave it running. There is no guarantee, and it only ever helps with stuck pixels; a genuinely dead one cannot be recovered. Do not press or rub the screen: the risk of damaging the liquid crystal layer further is far higher than the chance of rescuing one dot.

Because the browser reports CSS pixels, not physical ones. With display scaling at 150%, a 2560 × 1440 screen is described by the browser as 1707 × 960 with a pixel ratio of 1.5×. Multiply the two and you get the physical resolution - which is exactly the "Physical resolution" tile at the top of the page. That figure usually matches the panel spec on the box.

It is an approximation, which is why it carries a ≈ sign. The measurement counts how many frames the browser draws in a second, and that rate is locked to the sync signal of whichever display holds the browser window. So: drag the window to a second monitor and the figure follows that monitor; leave the tab in the background and the browser throttles drawing so heavily the figure becomes meaningless; a machine under load also reads low. For a reliable number, press "Measure again" a few times while the machine is idle.

That is the figure the browser declares, not what the panel can actually do. Most browsers return a fixed 24-bit as an anti-fingerprinting measure, even on a 10-bit per channel display. For the real answer, check your operating system's display settings or the manufacturer's specification.

Clean the screen first - dust and fingerprints look enough like dead pixels to fool anyone. Turn brightness up and switch off colour-shifting modes such as Night Light, Night Shift or True Tone, since they lay a yellow cast over everything and skew your colour judgement. For backlight bleed specifically, work in a dark room. Look square-on from about a hand's span away and sweep your eyes region by region rather than taking in the whole screen at once.

Yes, the colour fills cover the viewport on any device. The one exception is Safari on iPhone and iPad, which will not let any element go fullscreen, so the browser's address bar stays put and you cannot inspect the strip it covers. The workaround is to rotate the device and run through once more, which moves the covered area somewhere else.

There is no common threshold: every manufacturer sets its own, usually based on the number of faulty pixels and where they sit - a dot in the middle of the screen is treated more favourably than one near the edge. Many also distinguish bright dots from dark ones. Check your own manufacturer's policy, and if the device is newly bought, photograph or film the screen while a colour fill is up as evidence, as early as possible.

No. Reading the specifications and filling the screen both run in JavaScript inside your browser; the page sends no figures to any server and stores no results. Close the tab and it is gone.