src/language-html/parser-parse5.js/extendAst
function extendAst(ast) {
  if (!ast || !ast.children) {
    return ast;
  }
  for (const child of ast.children) {
    extendAst(child);
    if (child.attribs) {
      child.attributes = convertAttribs(child.attribs);
    }
  }
  return ast;
}