I'm OK with that. What does cross platform even mean? Windows, Linux, OSX? Maybe Android and iOS?
It's still easier than rewriting all of the development tools, libraries, etc in javascript and for the browser, and your app will be responsive and fast.
A single HTML documents will load on Win95 - Win8, old Mac Systems, OS X on PPC and Intel, a huge range of software and hardware running flavors of Unix, every smartphone OS ever, and be styled "natively" for every one of them (flat, shaded, rounded, even textured). That's only if you're coding for lowest-common-factor features, but that's not even possible for most other distributed files.
If you want to get fancy, and you're willing to sacrifice compatibility with less-cool platforms, JS is generally within 2x of native performance, which is plenty fast enough to feel "responsive". http://asmjs.org/faq.html
It's still easier than rewriting all of the development tools, libraries, etc in javascript and for the browser, and your app will be responsive and fast.