This implementation is incomplete in that we do not fully implement the
steps to match the given font against the fonts in the set.
This is used by fonts.google.com to load the fonts used for sample text.
There was no need to use FlyString for error messages, and it just
caused a bunch of churn since these strings typically only existed
during the lifetime of the error.
This is an ad-hoc implementation that resolves the ready() promise once
the document and all fonts collected by the style computer are done
loading. A spec-compliant implementation would include creating a proxy
CSS::FontFace for each @font-face and correctly implementing the
specification steps for font fetching, but we are far from there yet.
This hackish implementation should yield good WPT progress because it
will actually start waiting for the Ahem font to load before capturing
layout measurements. For example, it makes
https://wpt.live/css/css-grid/abspos/positioned-grid-descendants-001.html
go from 0/100 to 36/100 passing subtests.
