[css-fonts] Specifying changes to parameters for fallback fonts

[mozilla-apprentice]

Resolutions were made for csswg-drafts/#126.

[css-fonts] Specifying changes to parameters for fallback fonts

• RESOLVED: Solve the general case of fallback font adjustment via @font-face descriptors
• RESOLVED: Add advance-override descriptor to Fonts 5, precise details TBD

Discussion.

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