Firefox and Chrome seem to render box-shadow quite differently

Question

I was testing a box-shadow effect in both Chrome and Firefox and I was surprised to see a drastic difference in rendering between the two browsers. Notably, Firefox\'s rende

Answer 1

This is a long standing bug in Chrome which is fixed for Chrome 73 (coming March 2019).

https://www.chromestatus.com/feature/6569666117894144

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.

Note: This bug is older than forking Blink from WebKit. Safari ever had a different graphics engine.

https://bugs.chromium.org/p/chromium/issues/detail?id=179006

So the exact formula to preserve appearance through this change is
R' = 2 * (0.288675 * R + 0.5)

R' (new)     R (previously)
Chrome 73+   Chrome 72 and below
1.5px        1px
2px          2px
3px          3px
3px          4px
4px          5px
4.5px        6px

R' ≈ 0.7 * R       for R = 7px ... 12px
R' ≈ 0.6 * R       for R = 22px ... ∞