Don't add CSS scoping attribute to elements that don't need it

[Rich-Harris]

The other side of the #678 coin — there's no sense in adding a svelte-123xyz attribute to an element that doesn't have a corresponding style:

<div>
  <p>this should have the attribute, the parent should not</p>
</div>

<style>
  p {
    color: red;
  }
</style>