GNOME Devs' (that are also GTK devs) Motivation for Cross-platform GTK4 Apps?

As a GTK developer the answer is quite simple: There is no developer maintaining portability. To have a portable toolkit, you need at least one...

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@retr0id HTML is not a suitable replacement for GTK, Qt is the closest that exists
@lunch not suitable in what aspect?

@retr0id HTML is not and never was designed for highly interactive, dynamic, and accessible applications, and attempts to do so show how much it sucks with how much effort it takes to re-achieve thoss

HTML was designed for documents and that's all it should be used for