The Humble Border-radius
summary
Border-radius accepts up to four corner values and can take a slash to give different horizontal and vertical radii so percentages produce responsive ellipses; when adjacent radii exceed a side the UA proportionally scales all radii until they fit; the inner radius equals the outer radius minus border width so it can become zero or turn elliptical with unequal borders (inner radius = outer − border width); box-shadow follows curves, outline does not, radii are animatable, and Firefox reports used pixel values while other browsers return the specified percentages.
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