xmlSave() also can flush in some cases. When the encoding is not
available this can fail for short inputs, resulting in an empty string
which is interned but then wrongly tagged by RETURN_NEW_STR.
Fix this by checking the error condition and switching to RETURN_STR for
defense-in-depth.
This issue also exists on 8.3, but does not crash; however, due to the
different API usage internally I cannot easily fix it on 8.3. There it
gives a partial output.
Closes GH-18606.
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.
