I used to like the idea of (originally zope's) metal/tal templates. In theory the templates where valid html, complete with example values - and could be edited

I used to like the idea of (originally zope's) metal/tal templates. In theory the templates where valid html, complete with example values - and could be edited directly. Unfortunately this breaks down as soon as you start to break up the page into sections (you end up with a component that needs a "layout" to provide surrounding <html><body>(...) etc). Ironically, now with html5 being rather loose and fast, you might actually get a decent preview of such a widget again... But it's still not right for "web applications" as opposed to "hypertext applications". Full applications need full gui widgets - and html isn't a good fit.