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().
A string.
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"