Is it possible to access a closed #ShadowRoot on a non-custom element?

```
<div>
<template shadowrootmode="closed">
...
</template>
</div>
```

The only ways I'm aware of are `HTMLElement.prototype.attachInternals` which requires a custom element or to manually attach the root with `Element.prototype.attachShadow` and save the result. Are those the only two ways to get at a closed root?