Lets use two applications that are out on the market as examples.

Webflow: Produces good HTML and CSS but you still as the designer are expected to deal with ID's and classes directly. You also need to at least have a rudimentary understanding of css styling and the box model, designers are allergic to that.

Webydo: All design, but try to use the html inspector and check the code it produces. It is bloated. Check out one of the websites in their showcase, http://muccatypo.com/

One of the advantages using the canvas this way is that it allows the page to be rendered only through the use of numerical data saving bandwidth when delivering a webpage.