DocumentType: publicId property

The read-only publicId property of the DocumentType returns a formal identifier of the document.

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 "-//W3C//DTD SVG 1.1//EN".

const docType = document.implementation.createDocumentType(
  "svg",
  "-//W3C//DTD SVG 1.1//EN",
  "http://www.w3.org/2000/svg"
);

console.log(docType.publicId); // Displays "-//W3C//DTD SVG 1.1//EN"