XMLSerializer: Should prefer the default namespace to a prefix declared in an ancestor

[tkent-google]

Suppose that we have the following XML document, parse it, and serialize it.

<root xmlns:x="uri1">
 <table xmlns="uri1"/>
</root>

If we follow the current specification, the serialization result is:

<root xmlns:x="uri1">
 <x:table xmlns="uri1"/>
</root>

It's incompatible with Edge, Firefox, Safari, and Chrome 73-.
(Chrome 74 produces the above result, and we're fixing it.)

I think 12.1 in https://w3c.github.io/DOM-Parsing/#xml-serializing-an-element-node should be changed as following:

Original: Let candidate prefix be the result of retrieving a preferred prefix string prefix from map given namespace ns.

Proposed: Let candidate prefix be null if prefix is null and ns equals to local default namespace. Otherwise let candidate prefix be the result of retrieving a preferred prefix string prefix from map given namespace ns.

WPT domparsing/XMLSerializer-serializeToString.html contains a testcase for this behavior.
"Check if start tag serialization does NOT apply the default namespace if its namespace is declared in an ancestor."