Most purists insist on coding things in HTML/CSS and this stack is fairly unworkable for a good visual editor. Anyone who has built a robust visual editor ends up building one using canvas. (Figma is a great example)

Figma is great but it's not using classical tags to express fonts, tables, layout. Similarly Flash got it's layout and animation right... it just wasn't a web standard. A Canvas based editor/builder/WYSIWYG approach would be fantastic but it required us to move past HTML/CSS in the traditional and compatible model.

Said another way - if you try to use HTML/CSS to build a layout tool, you end up having to re-create browser layout and rendering engine.

Developers could build a design/tool similar to flash using canvas and I've actually prototyped elements of a tag/based design language that gets rid of the traditional HTML layout engine. It's significantly faster (1000x) to render/send over the wire but the moat of HTML/CSS is so large that I'm not sure there's interest in such approaches.