Unfortunately all impls compute different positions for text-top, and that the leading-trim: text value using it doesn't make this property interoperable. I would like the property to provide at least one value that is interoperable across platforms (browsers/OS).

I discussed with @litherum on this before. IIUC we would like to have two text-top/bottom metrics; one which is interoperable, another which is compatible with the current layout engine of the platform.

How about something like:

The em-height is a metric invented by Gecko, @jfkthame explained here.

@fantasai