Look into using your browser’s designMode functionality. It’s as WYSIWYG as anything can be. It’s great for editing HTML but not very suitable for writing HTML.

A more locked-down theming API could help. For example Firefox themes are always 100% safe to install. That said, Firefox themes are almost useless (they’re more like color schemes lol), and no one wants to lose KDE’s powerful customizability so 🤷🤷