Respectfully: To me these just sound like excuses.
I can write a web page that works correctly on all browsers. We all can. That web page won't do much of anything, but it's possible. So, there is a baseline "target subset of HTML/CSS" that gives you 100% coverage. From there, it's purely developer choice: When you add something, are you choosing technology that is widely available and supported, or are you choosing to throw 0.N% of users under the bus for some benefit (development speed/comfort)? Obviously, it's a trade-off, and no final product is going to work on 100% of configurations. All these choices deliberately made during development add up to the product you deliver at the end of the day. All I'm asking is that we recognize browser/platform incompatibility/inaccessibility as choices and not some inherent property of software.
When a developer says "it's too expensive to develop this for a dozen configurations" that just means they have already chosen to make their applications inaccessible, and are justifying it after the fact.
This goes to show you’ve never been anywhere near the actual development cycle of a real-world front-end web application. “So, there is a baseline "target subset of HTML/CSS" that gives you 100% coverage.” Oh really? Which subset? Which “HTML/CSS?” And 100%? Absolutely laughable.
The inherent property of software is that the only way to be sure your software works on a particular platform is to test on that platform.
There is not a baseline target subset of HTML/CSS that reaches 100% coverage that can be statically verified. HTML tables usually work in old browsers, but there were subtle bugs in old versions of Internet Explorer, bugs that you're especially likely to hit if you're using tables for layout (because you can't use modern CSS layout features). The only way to be sure that you didn't trigger one of those subtle bugs is to test your web app on ancient browsers.
The cost of reaching the last 0.N% of users rises with each platform you add to your test matrix. It costs money to test your web app on Internet Explorer. It costs even more money to fix bugs that only affect Internet Explorer.
I think you can't deny that doing that work is expensive. The question then has to be whether that work will repay itself somehow. But the last 0.N% of users will only provide ~0.N% increases to your revenue. Unless your revenue is astronomical, you can't afford even one full-time engineer to test and fix bugs on 0.N% of browsers.