[css-backgrounds] Specify exact pixel values for border-width: thin, medium, thick · Issue #7254 · w3c/csswg-drafts (original) (raw)
See this Twitter thread: https://twitter.com/tabatkins/status/1522261879762014223
Since browsers agree on 1px, 3px and 5px for border-width: thin, medium, and thick, respectively, it seems to me the spec could require this mapping for all UAs.