Ability to define placeholder for inner HTML

Is there a way to define something like:
```javascript
xtag.register('x-frankenstein', {
    content: '<div><p>component text</p>${innerHTML}</div>'
});
```
so it can be use in HTML file like:
```html
<x-frankenstein>
    <p>external text</p>
</x-frankenstein>
```
and then the rendered result to be:
```html
<div>
    <p>component text</p>
    <p>external text</p>
</div>
```
?