Find dead or stuck pixels, check colour reproduction, look for gradient banding, and inspect backlight uniformity and bleed — full-screen, entirely in the browser. Best viewed in a dark room. Wipe the screen first so dust isn't mistaken for a dead pixel.
Stays black on every screen, including white and the bright colours. It receives no signal and never lights up. Usually permanent.
Shows a fixed red, green or blue dot that doesn't change between screens. Often fixable — cycling colours, gentle pressure, or a pixel-exerciser can sometimes revive it.
On the smooth gradients, look for visible steps or stripes instead of a seamless fade. Heavy banding suggests 6/8-bit colour or aggressive compression; light banding is normal on 8-bit panels.
On grey, watch for clouding or patches that look brighter/darker. On the black screen in a dark room, glowing edges or corners are backlight bleed / IPS glow.