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.