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Pulsing Status Dot: The Ring Is a Box Shadow
A pulsing status dot is usually three elements: a wrapper, a ring, and the dot. It needs one. box-shadow paints outside the border box and takes no part in layout, so the halo is a second shape the element already owns: grow the spread from 0 to 14px while the alpha runs out and the ring swells past the dot, moving nothing around it. One span, two keyframes. The shadow reads color-mix(in srgb, var(--dot) 70%, transparent), so green, red, amber and blue all run the same pulse, and it inherits border-radius, so the circle and the rounded square differ by that one property. cubic-bezier(0, 0, .35, 1) throws the spread out at once and then crawls, which is what a ripple looks like; linear reads as a blinking outline.
animationcssbox-shadowno-jskeyframesstatuspulse