For the person who just downvoted me, I'd like to know why. Since when is making our opinions heard a bad thing on HN? I never said agree with me. Shouldn't downvoting be restricted to offensive posts?
I am not that person, but I can probably explain anyway.
The whole point of the web is to be a common runtime. You can use it from any browser and any device. You avoid the situation where you prefer browser X because it has some feature that is critical to you, but you have to use browser Y for some web sites. (So in the end, you'll use browser Y for all sites so that your user agent can actually be your agent by eg remembering your history/passwords. Or worse, you will be forced to use multiple browsers due to multiple incompatible websites.) You also avoid the situation where a new hardware platform or browser engine can never be released because there's no way to make it compatible with the current web.
In short, if a "web site" does not work in a strictly standards-compliant browser, then it's not part of the Web. It may be linked to from the web, and it may almost be part of the web, but it's really something else hanging off of it.
There are many such things. Flash apps are a common type. Heck, PDFs are too and until recently all videos were. There's nothing inherently wrong with them existing, as long as they aren't claiming to be part of the Web.
In this case, it may only be a QA shortcoming with respect to a 0.7% market share device. Inbox is not so benign, since the compatibility delay is a pretty strong indicator of corporate priority and policy.
And that's the danger, and the probable reason for the downvote. If you accept non-web things on the web, only thinking of the convenience and added functionality you gain (because your particular user agent is fine with it and makes it appear to be part of the web), then lockin will gradually set in, standards will lose their meaning, browser makers will start having to implement each other's half-baked experimental features that don't interoperate with everything else, cats and dogs will start living together, and there will be no reason not to gain the extra 10% in performance and functionality you get from a native app because your web sites don't run universally anymore anyway. So developers get to start maintaining half a dozen separate code bases because we've pissed away our opportunity to make a single codebase work everywhere. The blasé acceptance of that possibility is what earns downvotes.
The movement in this direction is already well under way on the mobile web, so I'm not just dreaming up imaginary hypothetical scaremongering.
