Is not about the first (and this is more about html/css than JavaScript). Is about the easier to work out, specially for custom styling and do custom widgets.
Not get me wrong, I don't like much JS/html apart as be hostage as everyone else to use it in the web, but is clear that as far to build the GUI is way more enjoyable than most GUI toolkit.
I think you need to prove that a (custom) UI is easier to build with HTML/CSS compared to QML. Or even plain old QWidget.
It can't be that web developers have been bitterly complaining about how poor HTML and CSS are at building UIs for years and years and suddenly it's become easy and enjoyable.
Actually it has become easy and enjoyable ;) Even for complex UIs.
Just think seriously about what a stack like es6/React/Redux brings on the table against plain old ES5, and you'll be struck by how well things have become.
Last time I implemented a complex web frontend was about 2 years ago. I can assure you that the process was anything but "easy and enjoyable". I will, of course, grant that things have improved greatly since the bad old days of IE6. But that's not a good anchor point to use for "easy and enjoyable".
