Support templated content

The template element in HTML 5 is special in the sense that it does not
add its contents into the DOM tree, but instead keeps them in a separate
shadow DOM

Support templated content

The template element in HTML 5 is special in the sense that it does not
add its contents into the DOM tree, but instead keeps them in a separate
shadow DOM document fragment. Interacting with the DOM tree cannot touch
the elements in the document fragment.

Closes GH-14906.

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