Agreed. I simply would not expect this to work properly if you don't specify a font. Just compare Times New Roman and Verdana, two fonts that are broadly available, and broadly different in terms of character widths. Verdana will probably bust out of that container.

Maybe if we had container queries, and could set font size as a…container unit? I don't even know what to call it. Like vw and vh units, but relative to a parent container.