This feature is intended to help Firefox frontend developers experiment with
replacing XUL content with modern flexbox. We might also eventually use
this emulation to *actually* render most or all of our legacy XUL UI.
MozReview-Commit-ID: 3g2W9o3t23H
--HG--
extra : rebase_source : a3e8b443d9b58e5af3287a23de6edc276ed5e847
It was exposed during stylo implementation but it should be behind
`layout.css.grid-template-subgrid-value.enabled` pref.
MozReview-Commit-ID: DqrU6zYgdES
--HG--
extra : rebase_source : 14859f5912faff9cf2daf9b10409114e853bab6d
Daniel Holbert
Nazım Can Altınova
Emilio Cobos Álvarez
Cameron McCormack
Xidorn Quan