The DOMImplementation.createDocumentType() method returns a DocumentType object which can either be used with DOMImplementation.createDocument upon document creation or can be put into the document via methods like Node.insertBefore() or Node.replaceChild().
createDocumentType(name, publicId, systemId)nameA string containing the name of the doctype, like html. Corresponds to the DocumentType.name property.
publicIdA string containing the PUBLIC identifier. Corresponds to the DocumentType.publicId property.
systemIdA string containing the SYSTEM identifiers. Corresponds to the DocumentType.systemId property.
A DocumentType.
const dt = document.implementation.createDocumentType(
"svg",
"-//W3C//DTD SVG 1.1//EN",
"http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd",
);
const d = document.implementation.createDocument(
"http://www.w3.org/2000/svg",
"svg:svg",
dt,
);
console.log(d.doctype.publicId); // -//W3C//DTD SVG 1.1//ENDOMImplementation interface it belongs to.