Using markup & stylesheets is important in any application development. For example, KDE uses KHTML for its markup, a fork of webkit. Most interface libraries are written in C/C++ and expose an API so that every developer who chooses to use it doesn’t go about re-inventing the wheel, like GTK.
Most operating systems have their own version of an application development kit. Useful as they might be, you still end up writing code and supporting multiple apps throughout the development process. Regardless, the important thing to remember is that the basics are the same as discussed in the case of GTK.
Luckily, there are a few of these tools that are featured prominently on the market that allow you to create write-once, run anywhere HTML5 desktop applications. Here is a quick rundown on some of the most noteworthy available frameworks:
You could just write a plain HTML5/CSS/JS app and ship that with the run time and add NodeJS on an as-needed-basis. Assuming your app is written with a modular architecture in mind, a desktop app could reuse a lot of components from your web app.
It should be noted that as an added perk, because you control the runtime environment of the application you can do a lot more with less. To add additional value, use the advanced HTML5 API as well as WebGL if need be, and don’t be afraid to use Flexbox for better regions and layouts.
Do look out for a Node-webkit desktop application from 3Pillar Labs all-nighter next!