[whatwg] supporting HTMLElement.dataset on SVG elements

[Cameron McCormack]

As part of aligning SVG with some not-so-HTML-specific features of HTML, 
the SVG WG would like to support data-* attributes on SVG elements.  In 
mixed HTML/SVG documents, it seems like you would want to be able to use 
these attributes regardless of whether the elements are HTML or SVG, or 
indeed MathML.

Does it make sense to move HTMLElement.dataset up to Element, so that 
all elements can benefit from it?  If not, could this attribute be split 
out on to a separate interface that we could have SVGElement implement?

(I'm also going to be looking at the other global attributes that HTML 
elements have to see if any make sense in SVG too.)

Thanks,

Cameron