This is a testharness.js-based test.
[FAIL] Document.replaceChildren() with an element, replacing an existing doctype and element.
  Failed to execute 'replaceChildren' on 'Document': Only one element on document allowed.
[FAIL] Document.replaceChildren() with a DocumentFragment containing a single element, replacing an existing doctype and element.
  Failed to execute 'replaceChildren' on 'Document': Only one element on document allowed.
[FAIL] Document.replaceChildren() with a doctype, replacing an existing doctype and element.
  Failed to execute 'replaceChildren' on 'Document': Can't insert a doctype after the root element.
Harness: the test ran to completion.