Description
https://drafts.csswg.org/css-font-loading/#dom-fontfaceset-readypromise-slot
https://drafts.csswg.org/css-font-loading/#switch-the-fontfaceset-to-loaded
https://drafts.csswg.org/css-font-loading/#fontfaceset-pending-on-the-environment
Per these definitions, the initial document.fonts.ready promise cannot resolve until "pending on the environment" is false. Making some assumptions about #1081 to clarify, this means that the promise should never resolve before the DOMContentLoaded event is fired.
However, as tested in Chrome, Firefox and Safari, the promise can resolve before anything else interesting happens: https://software.hixie.ch/utilities/js/live-dom-viewer/saved/4923
It might not reproduce with this test depending on network conditions, but I took it and appended a megabyte in a comment block and hosted it far away from me, and that made the promise resolve first in all 3 browsers. (Edge does not support the promise yet.)
Implementations do something different than the spec. We should figure out what it is and if that already happens to be interoperable, perhaps change the spec to say that.
EdgeHTML: @atanassov
Chromium: @kojiishi @irori @drott
Gecko: @dbaron @upsuper
WebKit: @litherum @youennf