There are already many HTML UI libraries/widgets that use custom attributes. Like setting the icon to use on a jQuery Mobile list item. Similarly, you've always been able to just set properties on them and call them or use them later. This just seems to add custom tag names instead of generic tag names and prototype functions instead of setup functions - syntactic sugar, basically. Unfortunately it has poor browser support, so there isn't much reason to use it rather than the more compatible traditional methods.
There are JavaScript libraries to bring it to the full set of browsers, so I guess if you really want to name your divs something pretty you can do so at the cost of performance and heavy scripting.