Recently the Twitter Bootstrap documentation gave a name to something that I've been excited about for a pretty long time: Markup API.
Markup APIs give superpowers to HTML.
Through the use of class attributes, data attributes, X-Tags, or other conventions they effectively extend the behavior of HTML, turning it into a kind of magic ink. Favorite examples of mine include Twitter Bootstrap, Wowhead Tooltips, and my own Instapoppin.
These advantages make it possible to create GUI affordances atop hand-coded HTML that make it much easier to write. As an example of this, I hacked up prototype slideshow demo and physics demo in July of last year. Dragging an element with the class
thimble-movable in the preview pane changes (or adds) CSS absolute positioning properties in the source code pane in real-time, and holding down the shift key modifies width and height. This allows users to size and position elements in a way that even a professional developer would find far more preferable to the usual "guess a number and see how it looks" method. Yet this mechanism still places primacy on the original source code; the GUI is simply a humane interface to change it.