[This issue has been annotated in the spec for awhile, but doesn't seem to have a corresponding GH issue, so filing one here.]

Both Thai and Hebrew are writing systems with strong top edges (similar to Latin/CJK). But while OpenType defines multiple top edge metrics (cap-height, x-height, ideographic, and hanging), none of these necessarily coincide with the Hebrew or Thai top metrics, which in a given font will often fall somewhere between the x-height and the cap-height, but not consistently the same place across fonts.

If initial-letter-align and text-edge are to treat all writing systems as equal citizens of the Web, we need metrics for them in OpenType, and we need values for them in CSS that will select those metrics.

Note: See also CSSWG OpenType liaison statement.