from @AmeliaBR

allowing number as a replacement for length happens automatically in all presentation attributes, see https://svgwg.org/svg2-draft/types.html#syntax. Explicitly allowing number in the CSS grammar allows it in the style property, too (this is needed for many SVG-specific properties from SVG 1). Since this [inline-size] is a new property, there is no legacy reason to allow plain number in the CSS property, it is probably best to harmonize with the definition in Logical Properties.