setInnerHTML for XHTML documents

When running under XML mode, normalize the incoming HTML in setInnerHTML.
@alexeyraspopov Well, this evaluates to `<br>` not `<br />` and hence still fails to validate as XML:

```
new DOMParser().parseFromString("<br>", "text/html").body.innerHTML;
```

Unless there is a way to cast a HTMLDocument into XMLDocument, serialization still seems required.