How else, you ask? As a percentage of the background clip box would be the obvious possibility. It’d be less useful, but it’d be arguably more consistent with p

How else, you ask? As a percentage of the background clip box would be the obvious possibility. It’d be less useful, but it’d be arguably more consistent with percentages just about everywhere else. (Though, being based on the element’s background clip box rather than its containing block’s content box, it would still be somewhat different.) It’s common for people to wish that percentages worked this way with other properties, because in some cases that’d be useful, but they don’t. (Example: margin-based centring, `width: 20em; margin-left: calc(50% - 10em);`.)