When we add a Shadow DOM to replaced elements (e.g. img, input, iframe, etc), we would like to style the inner element of them. However, there is no standardized way to style them.

For example:
  We can easily style an img element if it does not have an AuthorShadowDOM.
    img {
        width: 100px;
        height: 100px;
    }

  Then we add an AuthorShadowDOM for an img element, and let the innerHTML of the AuthorShadowRoot be "<div style='margin: 10px'><shadow></shadow></style>". Since the width of the img element is 100px, and the inner element has 10px margin. It's OK. But how do we set the width of an image itself? 

In WebKit implementation, we have pseudo id of such element. So we can style it anyway.
However, when it comes to the spec, we should have some standardized way to style them, I believe.