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.