CSS has too much old crap, how many ways you can center a thing? How many ways you can horizontal align some stuff? There should be only 1 way and that way should not have hacks like use negative margins.

What a Qt or other similar frameworks gives you is consistency, all components in all apps will look and work the same (with the exception of customized ones). This means you can focus on UX and not on bad design. My experience is that some bad designer will force his opinion on the users, will force his preferred fonts, font sizes and font colors on the users(making text hard to read), will disabgle text selection for some weird reason, will fuck with the scrollbars because the native ones are ugly etc.

For a working app that is not a music player you don't need the "power" of css, say you seen video games have a small Launcher/config window that hase buttons,drop downs, checkboxes , I noticed those use Qt or Windows Forms. I am also noticing that modding tools , open source tools (that are not GNOME) also focus on functionality and you will not see buttons with round corners, fancy fonts and animated borders.

IMO Electron advantage is not his the "powerfull"css and html and it is that you can reuse the existing node ecosystem and existing web developers and the alternatives are also lacking for higher level languages (GTK is shit IMO)