Sure, and the grit of clay at a specific location is arbitrary too, but if that’s the material you have to work with, then that’s the material.
I think you’re being too literal about what constitutes material. You think only something the level of hardware or a programming language is “material” and everything above that is arbitrary, but I would argue ALL of the layers of the stack constitute material properties.
How about this: form submission. You don’t need it... you can use Ajax. But forms that can be submitted will work in more environments (no JavaScript, forward and back navigation, etc)
You can fight that current, but the underlying technology wants you to use forms.
I’m not really arguing about Brutalism though, so much as modernism. A modern web page would contain a minimum of changes to what the basic simplest HTML would yield. You start from the raw material with all its idiosyncrasies and you only add a dimension of “designed stuff” when you have a specific articulable rationale for it.
(This is totally different from the colloquial definition of “modern web applications” which are anything but respectful of their materials)