It was, at one point, but now that all of the browsers autoupdate and mobile device lifespan is about 2 years, it's rare for anyone to test on anything very old. Try submitting a bug to a site that things don't work on IE5 or NN6.
Also wrong, most browsers stopped supporting <blink> and <marquee> years ago (as well as many other outdated features, or plugins such as Java or Flash)
I want to say those are bad examples but nonetheless valid. A better expample would be Javascript's strict-mode situation. In less civilized times, you could do all sorts of terrible shenanigans with javascript, and luckily for developers strict-mode gained support to the point where everyone naturally assumed you were talking about a strict environment. When ES6 came around, those 'sloppy' mode differences made implementing new features tough, so with adopting ES6, 'sloppy mode' was implicitly dropped.
So yes, the web isn't by any means backwards compatible-- but definitely better at it than most ecosystems.
Yes, and the <bgsound> tag as well. What's your point? We're talking about a tiny part of the spec, vs. introducing something like QML to the ecosystem.
It was, at one point, but now that all of the browsers autoupdate and mobile device lifespan is about 2 years, it's rare for anyone to test on anything very old. Try submitting a bug to a site that things don't work on IE5 or NN6.