the comments here informing us they default to behaving like <span> instead of like <div> is the biggest disappointment of my day so far

I think it makes sense that if HTML detects an unknown element that it doesn't give it block styling automatically. Only if you specifically specified it for that type of element.

Inline styling is kind of the default in HTML.

See my other comment: you can set default styles using

:where(:not(:defined)) { display: block }