Historically, Blink's blur-radius interpretation has been at odds with both the CSS and Canvas2D specs: Blink shadows cover about half the expected area (see linked bug). With this change Gaussian blur sigma is now computed as 1/2 blur-radius, as mandated by spec. Blink's shadow implementation now matches FireFox and Safari.
Docs: https://www.w3.org/TR/css-backgrounds-3/#shadow-blur https://html.spec.whatwg.org/multipage/canvas.html#when-shadows-are-drawn