You can create custom widgets like some fancy buttons in Qt too but the appeal is to not customize things and let the user platform and theme choice to dictate how things look like.

Qt had layouts years before html got the flexbox and gridbox too so I don't think there is a feature in html that is missing in Qt, Qt also includes QtWebkit if you ever need say to display some html or run some JS so basically you have access to native libraries and to JS/HTML if you need.