1. DOM Parsing and Serialization

    Various DOM parsing and serializing functions, specifically `DOMParser`, `XMLSerializer`, `innerHTML`, `outerHTML` and `insertAdjacentHTML`.

  2. element api: outerhtml

  3. element api: outerhtml: requires `trustedhtml` instance when trusted types are enforced

  4. element api: outerhtml: serializes `<` and `>` in attributes as `&lt;` and `&gt;` (see this spec issue)