CFR ascender and descender

[Chapman, Christopher]

Hi Folks,
I have a question about the CFR ascender and descender metrics.
The CFR spec. has the following descriptions of ascender & descender:
    ascender = "string"
    Optional. The horizontal typographic ascent expressed as an integer.
    descender = "string"
    Optional. The horizontal typographic descent expressed as an integer.
My understanding is that if these optional attributes are missing from the CFR they are to be inherited from the first component font specified by either 'ComponentDef' or 'LanguagePreferredList' element.
The OpenType spec. has the following possible sources of the ascender and descender metrics:
    hhea.Ascender & hhea.Descender
    OS/2.sTypoAscender & OS/2.sTypoDescender
    OS/2.usWinAscent & OS/2.usWinDescent