It's hard (not impossible) to make it work in principle because a printing press doesn't need to infer meaning from the post script or whatever you provide it. A browser does need to, so you'd need to layer the extra semantics on top of your WYSIWYG editor.

Which means asking questions like "this is 36 pixels high: is it a heading,; or is this just a landing page with big text?" etc. So maybe I lack imagination, but I don't see it being that easy a problem to solve from a UI point of view.

There are plenty of semantics in InDesign documents. InDesign is all about character, paragraph, list and table styles – it’s what you use to make InDesign work. You wouldn’t have to add much to make those styles do all the semantic work.

Yeah, you would still be able to just ignore all that – just like you can adjust each headline individually in InDesign – but that would only mean that you are an incompetent user of the software. A good WYSIWYG editor wouldn’t always have to produce great markup but a competent user of the software should be able to easily make it produce great markup.

I’m really not that sure why adding a GUI for semantics would be so hard, especially since existing concepts like paragraph or list styles already map very well to HTML concepts.

One big part is that when something is designed for print, it is essentially always designed for a fixed size page, so you don't have to deal with resizing. Then there is the fact that web sites have to cope with people not always having the same fonts you do - which makes it almost impossible to do pixel perfect designs involving text rendering.

Then there is the fact that something like an Indesign document doesn't really have any concept of separating the style from the actual content. For the time being at least, humans are much better at logically ordering and marking up text, and then adding the style with CSS, unlike programs which add a whole lot of extra markup and CSS to do the same thing.

Those are just a few things which make WYSIWIG work a lot better for print than it ever could for web - and there are more... These things will probably be somewhat resolved as time goes on (ie. with wider support for @font-face and so on) but I think that hand written HTML will be better for a long time to come.

I’m not suggesting to slavishly copy InDesign and I’m not saying it’s easy. InDesign would be absolutely horrible for web design. I’m merely suggesting that WYSIWYG for HTML and CSS is not impossible.

Being able to flexibly test (e.g. different fonts, different resolutions, different browsers) would certainly be one of the requirements, as would be the implicit assumption that pixel perfect designs are not possible.

As for style and content? InDesign does actually separate them to a degree. It’s not perfect but neither is HTML at that task. The concept of character, paragraph, list and table styles is central to InDesign. It’s what makes InDesign so great. It even has a bare bones text input mode where all you do is type completely unstyled text. It’s then easy to apply all the different styles you made to your text.

I think a truly powerful WYSIWYG HTML editor is eminently possible, someone has only dare to do it.