[css-inline] Specify fallback baselines estimations for text?

In the CSS spec, hanging baseline is defined as the following Corresponds to hanging baseline from which characters in Tibetan and similar unicameral scripts with a strong but not absolute top edge seem to “hang”. Assigned to hang in OpenType and to bsln value 3 in TrueType AAT.

It didn't describe what's the fallback value for hanging baseline if "hang" is missing in OpenTyoe table. The definition is not entirely clear as well, it only says a strong but not absolute top edge. So it allows browsers to have their own interpretation of the baseline value if "hang" is missing in OpenType table. I am wondering if we could add a note to suggest how this value could be computed from font ascent and font descent when it's missing.

ideographic baseline has the same issue.

Start with the css-inline hanging-baseline section and the linked ideographic baseline definition. Compare their OpenType and TrueType references, then determine whether the specification should define fallback estimations for missing baseline data; done means the fallback behavior and the hanging-baseline terminology are explicitly resolved in the spec.