[cssom] style attr declarations undefined

[gsnedders]

https://w3c.github.io/csswg-drafts/cssom/#the-elementcssinlinestyle-mixin

The style attribute must return a CSS declaration block object whose computed flag is unset, whose parent CSS rule is null, and whose owner node is the context object.

This doesn't actually say what the declarations of the CSS declaration block are, and the definition of CSS declaration block doesn't give any default value.