We should throw out box-shadow and text-shadow properties when the blur radius is negative; the rule should be considered invalid.
https://wpt.fyi/results/css/css-backgrounds/box-shadow-radius-001.html?label=master&label=experimental&aligned&q=shadow-radius It has box-shadow negative values coverage. Ignore my comment, if I am wrong. Thanks!
Created attachment 464659 [details] test for negative values in box|text-shadow The negative values in these tests are the spread-radius (4th value), not the border radius. Negative values are authorized on the 4th value. https://developer.mozilla.org/en-US/docs/Web/CSS/box-shadow box-shadow: offset-x | offset-y | blur-radius | spread-radius | color 3rd <length [0,∞]> Specifies the blur radius. Negative values are invalid. If the blur value is zero, the shadow’s edge is sharp. Otherwise, the larger the value, the more the shadow’s edge is blurred. See Shadow Blurring, below. — https://w3c.github.io/csswg-drafts/css-backgrounds/#box-shadow https://developer.mozilla.org/en-US/docs/Web/CSS/text-shadow text-shadow: offset-x | offset-y | blur-radius | color OR text-shadow: color | offset-x | offset-y | blur-radius https://w3c.github.io/csswg-drafts/css-text-decor/#text-shadow-property doesn't really forbid the negative value apart a reference to the length where it says it can be problematic. The story is slightly different in text-shadow 4 https://drafts.csswg.org/css-text-decor-4/#text-shadow-property where it explicitly reference the box-shadow way of doing things. https://drafts.csswg.org/css-backgrounds-3/#typedef-shadow I uploaded a test. There's no styling going on when the values are negative. So probably this has been solved in the past. And we can also see this in the Web Inspector.