In my experience, unfortunately it isn't true that apps work with very high reliability. This is why I use multiple browsers. I don't want to use Chrome, and I encounter new websites with functionality that don't work properly in Safari or Firefox quite regularly. Some of this is even quite basic functionality.
From things like shopping carts where the final payment button doesn't work in Firefox (nothing happens), to video conferencing that sort-of works in Safari but then the audio breaks while it is reliable in Chrome.
Even GitHub doesn't render properly in Safari occasionally (the page is full of weird size text and giant rectangles). But that's almost certainly a Safari bug.
This is exactly the problem I was suggesting might be improved by an increase in browser diversity. 9/10 times, the issue isn't the browser itself, the issue is that the dev didn't bother to test and find the (handful of often small and easy to fix) bugs in other browsers. If they were forced to do QA - and ideally, had to adhere better to the actual standards to make their own jobs easier - you might have an easier time using those apps (and the libraries they depend on) in other browsers.
I think this is more a problem with developers working in Chrome and using Blink-specific features. I stopped having compatibility issues with my apps when I moved to Firefox years ago.
Some of it is Blink-specific, but some of it is silly Javascript stuff. How else do you explain a shopping cart final payment button not working.
I like development in Firefox too. When I was doing it intensively (just ~2 years ago), I found some of the CSS I was using behaved differently between Gecko, WebKit and Blink, though (and differently again in Android's pre-chrome browser), so I reconfirmed that I do need to test some things across all browsers. By differently, I don't mean prefixed browser-specific CSS (although there is that), I mean standardish things like table row height calculations showed up significantly differently, certain border highlights, and a number of WebKit rendering bugs that Gecko did not have. Key-event based text editing and wheel events are also significantly different, even more so when you include the common mobile browsers.
In my experience, unfortunately it isn't true that apps work with very high reliability. This is why I use multiple browsers. I don't want to use Chrome, and I encounter new websites with functionality that don't work properly in Safari or Firefox quite regularly. Some of this is even quite basic functionality.
From things like shopping carts where the final payment button doesn't work in Firefox (nothing happens), to video conferencing that sort-of works in Safari but then the audio breaks while it is reliable in Chrome.
Even GitHub doesn't render properly in Safari occasionally (the page is full of weird size text and giant rectangles). But that's almost certainly a Safari bug.