Comment by chrismorgan

Stroke expansion is a complex topic, with more than one reasonable result (subjective preferences), and a whole lot of corner cases and incorrect answers.

Firefox has chosen to expand based on distance at all points, which is one of the reasonable answers and probably the most general one; a cusp then expands to a curve.

The others have chosen to retain cusps, which can be a reasonable answer and I believe is a lot cheaper to compute; but degenerate cases abound as you expand past the feature size (distances between nodes), so that by the fourth red ring it’s obviously incorrect.

Box shadows are another case where expansion occurs: the fourth length parameter, spread distance. If the corner is a cusp, the shadow corner will be a cusp. If it’s rounded, the shadow corner will be rounded. See https://drafts.csswg.org/css-backgrounds/#shadow-shape for some helpful diagrams. A sneaky trick: .1px border-radius means the box still looks square, but the expanded shadow will curve. Sometimes useful. But back on the original content of the article—if you use a font with microscopic curves instead of cusp nodes, Chromium/Safari will look more like Firefox.