[whatwg/dom] Don't store is value when it won't be useful (#661)

Fixes #566.

I believe the impact of this is entirely on the serialization of elements created this way.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/661

-- Commit Summary --

  * Don't store is value when it won't be useful

-- File Changes --

    M dom.bs (9)

-- Patch Links --

https://github.com/whatwg/dom/pull/661.patch
https://github.com/whatwg/dom/pull/661.diff

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