Okay, let's flip it: why would Apple, Microsoft, etc.. agree with such a law? What would the trickle down be for browser makers and website creators?
Hypothetically, preemptive market recapture? This could theoretically make foss OSs non-kosher for any suitably large market (websites, games, chat platforms, etc), and serve to force foss OSs into a niche where theyre technically capable of being personal desktop software, but practically unusable because of lockout.
This is part of a large movement to end personal computing as we know it, and give all control over our digital lives to the mega corporations. It is already impossible to use some national ID or banking apps on non-Apple and non-Google phones. (say GrapheneOS), giving them an immense edge in the market, and they're using that domination to add more restrictions like installing non-play store apps on Android. This goes bit by bit, try to add a restriction, go back enough to quiet the backlash, try again. This new law is a foot in the door to achieve the same restrictions on PCs. PCs are the last open computing platform and bastions of personal digital freedom and thus they must be destroyed by capitalists and governments. Once the technical means have been put in place to restrict access to some services, the system can be hardened : impose that the age checks can't be tampered with by using a TPU module, impose a link to the individual's identity so that all online activity can be traced to anyone. Pretenses are easy to find : protect children, block sex offenders from accessing dating sites, and so on. At this point I'm pretty convinced we're geared towards a global totalitarian regime, and there's a lot of evidence.
Doesn't Apple already ask you for your age when you set up a new device? And I think Microsoft requires you to be 18 on your admin account.
It's a classic case of regulatory capture. Only the biggest players can get away with fully implementing age verification — if Microsoft requires age verification to use its OS, most people will oblige. If HN starts doing that most people will probably just log off forever.
What is maddening is how often people think such laws are about limiting the influence of "big tech." Big tech isn't going anywhere because of these laws, you are.