But people ship python software, just like they ship CSS software, and python is bundled in many operating systems. When somebody ships e.g. a CLI tool to manipulate subtitle files, and it uses a language feature from python 3.9, that somebody is excluding you from running it on your 11 year old system.
People get new browser versions for free, there are more important things to thing about than users that for some reason don‘t want to upgrade. Like I would rather have my layout done quickly with nice elegant code (and no hacks) and spend my extra time developing an excellent UX for my users that rely on assistive technology.
Note that your wish for stabilization was delivered by the CSSWG with the @supports rule. Now developers can use new features without breaking things for users on older browser. So if a developer wants to use `display: grid-lanes` they can put it in an @supports clause. However if you are running firefox 45 (released in May 2016; used by 0.09% of the global users) @supports will not work and my site will not work on your browser. I—and most developers—usually don’t put things in an @support clause that passes "last 2 version, not dead, > 0.2%"
People get new browser versions for free, there are more important things to thing about than users that for some reason don‘t want to upgrade. Like I would rather have my layout done quickly with nice elegant code (and no hacks) and spend my extra time developing an excellent UX for my users that rely on assistive technology.
Note that your wish for stabilization was delivered by the CSSWG with the @supports rule. Now developers can use new features without breaking things for users on older browser. So if a developer wants to use `display: grid-lanes` they can put it in an @supports clause. However if you are running firefox 45 (released in May 2016; used by 0.09% of the global users) @supports will not work and my site will not work on your browser. I—and most developers—usually don’t put things in an @support clause that passes "last 2 version, not dead, > 0.2%"