document-outerhtml/src

import doctypeToString from 'doctype-to-string'

export default function documentOuterHTML(document) {
	if (!document
		|| document.nodeType === undefined
		|| document.nodeType !== document.DOCUMENT_NODE
	) {
		throw new TypeError('Expected a Document')
	}
	const html = [...document.childNodes]
		.map(node => nodeToString(node))
		.join('\n')
	return html
}

function nodeToString(node) {
	switch (node.nodeType) {
		case node.ELEMENT_NODE:
			return node.outerHTML
		case node.TEXT_NODE:
			return node.textContent
		case node.COMMENT_NODE:
			return `<!--${node.textContent}-->`
		case node.DOCUMENT_TYPE_NODE:
			return doctypeToString(node)
		default:
			throw new TypeError(`Unexpected node type: ${node.nodeType}`)
	}
}