Sciter's author here.
> HTML/CSS based native interfaces are some of the hardest to use, least reliable, least accessible, least robust interfaces I encounter
This has nothing with HTML/CSS per se but rather questions to particular products. Same complain may apply to any UI.
> The best native apps look and behave natively;
"Native app" term these days is something almost non existent.
In these terms Microsoft Office is not a native app as they use custom UI framework for UI. Pretty much whole Window 10 UI now is not "native" in this sense. UWP is a custom UI layer that uses windowless DirectX based UI.
There are some low level utilities left in dusted corners of Windows that can be classified as "native" but there are just few of them - no one cares.
Consider my https://html-notepad.com as an example (purely sciter app and so HTML/CSS UI), this application simply cannot use "native UI". No OS has needed components out of the box.
Yet pretty much any modern app must have one way to another to render and to produce HTML - we want our applications to be connected (read: interact with the Web).