As a web developer, I agree

But ensuring full compatibility with all three major engines (those being gecko, blink and webkit) is unnecessarily hard, as they have their own subset of features

For example: Webkit does not support extending built-in HTML elements using WebComponent, but Gecko and Blink do support this feature. Or Chrome being the only browser that fully supports the View Transitions API. Or webkit's CSS vendor prefixes

The list goes on and on.

You could fix most of these issues by providing polyfills, but that increases the amount of files that you have to load in order to make a feature work on other browsers.

If only there was some sort of standard... Oh wait, there is one, W3C. Idk what they are doing tho.