It's not an either-or proposition. There's a spectrum of things that are supported to varying degrees by various browser platforms and a spectrum of degrees of

It's not an either-or proposition. There's a spectrum of things that are supported to varying degrees by various browser platforms and a spectrum of degrees of caring about compatibility. On one end of the spectrum, you might have questions about supporting IE9 (i.e. EOL software that the maker no longer updates but that you as a web dev might have to care about), on the other you might have questions about supporting untranspiled JS class properties or top level await (i.e. software features that you as a web dev may feel entitled to use despite knowing about limited adoption). It can be argued that browser vendors ought to do their best to align with standards, while simultaneously arguing that old devices do exist in the wild and some amount of onus falls on web developers to cater to segments above some threshold of usage.