Every time I get that much closer to using a webpage as an application of some kind, cold hard reality leaps out of the closet and tears my head off.
Movable Type 5's default in-browser rich text editor is pretty lousy, so I've tried customizing it or replacing it. Every since one of these attempts has ended in failure, because it is next to impossible to precisely define the behavior you want in a browser as opposed to what you get in something like Windows Live Writer.
WLW can intelligently strip pre-formatted text so that only the most crucial tags are preserved. Paste a quote from a webpage into an RTE and you get a horrible mess of tags that have to be cleaned up manually.
The default RTE in MT doesn't even let you remove a hyperlink.
It doesn't support Ctrl-key actions for things like bold or ital, either. That, you have to add manually — and good luck getting that to work in more than on browser at any given time.
Just try and insert something as simple as a <BLOCKQUOTE> tag in Chrome. No! Why settle for something as simple as <blockquote> when you can get <blockquote class="webkit-indent-blockquote" style="margin: 0 0 0 40px; border: none; padding: 0px;"> for FREE!!? (Even if they, you know, clash with my actual page stylesheet.)
I could go on. My point is that when you run a webpage as an "app", especially a page with an advanced degree of human interaction like an RTE, you're totally at the mercy of the browser's interpretation of the RTE. And since no two browsers are different — and sometimes they're not even the same after a point revision (I'm looking at you, Chrome), you get wildly unpredictable results all the time.
I've since gone back to using WLW, which may require a separate install and whatnot, but I will gladly pay that price in exchange for getting something which works and which has consistent behavior.
I guess it all comes down to which you value most: consistency or convenience? I don't have to think very hard about that one.