Even if it was some other vendor the fact that it's non-standard isn't great.

The "vendor" in this case is GCC and there are plenty of non-standard GCC extensions in use today. The Linux kernel standard gnu89, not C89, after all. I doubt you can even compile a usable Linux kernel sticking purely to the official C standard.

The same tricks are also enabled in the plan9 extensions, but enabling plan9 extensions also enables a bunch of other tricks and those changes landed later than the Microsoft ones. Aiming to enable plan9 instead probably could've saved the Linux kernel half a decade of "Microsoft bad" comments, though.

I think it's been gnu11 for a few years now.

Ah, you're right, I guess I must've been directed to an outdated documentation site when I looked up the exact configuration Linux uses.

Still, it's not the official C standard, but a specific flavour of C11, so my point still stands.

border-box says hi [1]

[1]: https://www.paulirish.com/2012/box-sizing-border-box-ftw/

(Funnily, tables always default to border-box, so the objections in CSS standardization at the time is really silly.)