Then it works in two browsers: most popular and their own. Ideally there would be such a large number of in-use standards compliant browsers that companies had to comply.
The problem isn't so much that browser vendors don't adhere to standards, it's that they make their own additional features and then make webpages that work with those. Having standards won't do anything for that, unless the standard says "you must not implement extra functionality".
There are four or five out there. I would expect the average modern web app to be compatible with Chrome, Safari, Firefox, Edge, and possibly IE11 if that’s appropriate for the particular use case.
