It seems a primary reason for unitless is inheritance behavior,
https://developer.mozilla.org/en-US/...SS/line-height
That's not really applicable to my use case here. In any event, the point was using line-height with p wasn't working with any variation of line-height ("I've tried large and small numbers with em, unitless and %, but it always looks the same.")
I continue to think different treatment of paragraph and heading tags with respect to whether CSS line height has an affect makes little sense.