findPrefixedAttrs = (node, prefixes, attr) => {
  return prefixes
    .map((prefix) => `${prefix}:${attr}`)
    .filter((attr) => node.attributes[attr] != null);
}
@param {import('../lib/types.js').XastElement} node
@param {ReadonlyArray<string>} prefixes
@param {string} attr
@returns {string[]}