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.

But again you're flattening all browser compatibility into ancient browsers that 10 people use and saying closing the end of that gap is far too difficult to justify the time and expense required, but what exactly are we talking about there? What broke? Can people on IE6 get the majority of the content but the subscription popup is broken, or does the page fail to render entirely and leave them completely high and dry?

It's impossible for me to engage with this thought experiment without thinking of hundreds, perhaps thousands of sites I've been to (their provider rhymes with Rare Mace) where literally nothing works without JavaScript, and I don't mean animations are broken or images look funny, I mean the website is a white fucking screen because literally everything is loaded in via esoteric new JS frameworks which aren't firing because the engine choked on an analytics package and died before it even got that far, and that site is showing...

... text. Formatted text. With perhaps some pictures. And animations nobody outside of marketing cares about.

So like, is your site broken because it's legitimately cutting edge shit, doing difficult work, and providing an answer to a complex user problem? Okay cool, IE6 support is probably not a high priority, I agree. Or, is it an utterly run-of-the-mill ad for your company's services, that was made incorrectly by people who don't know what they're doing, and/or have overengineered it beyond recognition of the actual problem it was trying to solve? If it's that one, then put your shiny toys down, rebuilt it simply and with regular tooling, and THEN see how your IE6 compatibility is doing.

I'll tell you this much: I've NEVER tested for IE6 on my personal website. I just did. Navigation is a bit wonky and my blur filter effects are broken, obviously. But you can still read my posts and navigate about.