Explain how to handle the "dtls" and "flac" OpenType features

[fred-wang]

From https://docs.microsoft.com/en-us/typography/opentype/spec/math

flac Flattened Accents over Capitals
This feature provides flattened forms of accents to be used over
high-rise bases such as capitals.
This feature should only change the shape of the accent and should
not move it in the vertical or horizontal direction. Moving of the
accents is done by the math handling client
Accents are flattened by the Math engine if their base is higher than
MATH.MathConstants. FlattenedAccentBaseHeight.

dtls
Dotless Forms
This feature provides dotless forms for Math Alphanumeric
characters, such as U+1D422 MATHEMATICAL BOLD SMALL I,
U+1D423 MATHEMATICAL BOLD SMALL J, U+1D456
U+MATHEMATICAL ITALIC SMALL I, U+1D457 MATHEMATICAL ITALIC
SMALL J, and so on.
The dotless forms are to be used as base forms for placing
mathematical accents over them.

There were experiments in Gecko 6 years ago and dtls is implemented:
https://bugzilla.mozilla.org/show_bug.cgi?id=963079

[fred-wang]

IIUC, we decided to move advanced OpenType features to level 2.