Re: does <set> add an attribute to node.attributes

Just to clarify, not changing the DOM attribute values does seem to be
normative. From SMIL 3.0:

  The presentation value which results from applying an animation is
not visible through the DOM.[1]

(That said, elsewhere there appear to be concessions for
implementations where that is difficult.[2])

However, SVG has added interfaces for querying these values.

That, however, means that creating a snapshot does take a fair bit of
code as one needs to go through SVG's animated type interfaces rather
than just hasAttribute/getAttribute. That might be unfortunate, but
it's the case regardless of whether the attribute is created or not.

[1] http://www.w3.org/TR/SMIL/smil-DOM.html
[2] http://www.w3.org/TR/SMIL/smil-animation.html#animationNS-AnimationSandwichModel
"In some implementations of DOM, it may be difficult or impractical to
maintain a presentation value as described..."