Some cases when document.documentURI is available but not document.URL

Question

While going through the location sources for DOM XSS attacks, I was trying to understand their differences in their usages(i.e where to use what). According to MDN, the difference between document.documentURI and document.URL is following.

HTML documents have a document.URL property which returns the same value. Unlike URL, documentURI is available on all types of documents.

Can anyone explain the cases where only documentURI is available and not URL?

EDIT:

I would like to know a few specific cases where this actually happens.

Answer 1

document.documentURI is available on HTML documents and on non-HTML documents.

document.URL is available on HTML documents.

---

Example:

<?xml version="1.0" ?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN"
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg version="1.1" xmlns="http://www.w3.org/2000/svg">

  <circle cx="250" cy="250" r="50" fill="red" />

  <script type="text/javascript">
  <![CDATA[
  alert(document.URL);
  alert(document.documentURI);
  ]]>
  </script>

</svg>

The first alert (document.URL) will be undefined.