If you're talking about fonts rendering differently, or some line being some pixels further to the right, same thing; incomparable.
If you're talking about corporate web apps written for IE, those aren't accessible from a phone anyway, so the distinction between native and web app is meaningless for them.
If, instead, you're talking of websites that really don't work on a modern browser, and you stumble on them day to day, you just have an experience that's different from most other people. The easiest way to tell that what I'm telling you isn't just my individual experience is to compare what you see and read today from what used to be the case in the days of IE domination.
Acting like a dick that thinks people that disagree with you are in a sorry little bubble of ignorance is just a character flaw; nothing to do with this.
It's not comparable to writing multiple native apps, but it's the exact same model as having to use cross-platform toolkits like QT. The web has just replaced OS with browser.
> taking about fonts rendering differently
This mimics what happens when using something like QT or Java since they at least try look kind of like the platform they're running on.
> talking about corporate web apps written for IE
IE & Chrome specific features are exactly kinds of things that make cross-browser development just like cross-platform development. Your site either has to use the lowest common denominator or be littered with platform ... err browser specific code -- exactly like native apps.