Prefix collision issues in serialization algorithms

While revisiting the XML serialization spec I found a few more bugs in it that can lead to serialization issues:

prefix redeclaration confusion

const root = document.appendChild(document.createElementNS('ns1', 'pre:root'));
const child = root.appendChild(document.createElementNS('ns2', 'pre:child'));
child.appendChild(document.createElementNS('ns1', 'grandChild'));

This currently results in grandChild being assigned the pre prefix without redeclaring it, which places it in ns2 instead of ns1.

Firefox assigns a new a0 prefix to child here to avoid this. Chrome redeclares the pre prefix on grandChild back to ns2.

Chrome's behavior seems to more closely match the author's intent in that it preserves all prefixes as specified. To match that, the algorithm should likely keep track of the prefix to namespace mapping in addition to the currently tracked inverse mapping, and use that to check if the prefix it wants to use needs to be redeclared.

generated prefix collisions

const root = document.appendChild(document.createElementNS('ns1', 'ns1:root'));
root.setAttributeNS('ns2', 'attr', 'value');

This currently results in two declarations for the ns1 prefix on the root element, one for ns1 produced as part of serializing the element, one for ns2 produced for the generated prefix ns1. That is not well-formed.

Firefox and Chrome both assigns a new prefix to attr here to fix this. In Firefox the prefix is a0, Chrome uses ns2.

To fix, the "generate a prefix" algorithm should probably check whether the prefix it intends to generate isn't declared already. If there is an existing local declaration for the prefix, it can loop to try a higher prefix index, matching Chrome's behavior.

Start with the XML serialization spec's prefix-redeclaration and generate-a-prefix algorithms. Reproduce both examples from the issue, then verify that prefix-to-namespace mappings and generated prefixes avoid collisions while preserving the intended namespaces; the issue is complete when both cases produce well-formed serialization.