DocumentType: systemId property

The read-only systemId property of the DocumentType returns the URL of the associated DTD.

For synthetic DocumentType, this property reflects the value given in parameter to DOMImplementation.createDocumentType().

For HTML documents, browsers always set it up to html, whatever the actual DOCTYPE in the source code is. For SVG documents, for example, it can be "http://www.w3.org/2000/svg".

const docType = document.implementation.createDocumentType(
  "svg",
  "",
  "http://www.w3.org/2000/svg"
);

console.log(docType.systemId); // Displays "http://www.w3.org/2000/svg"