Document `interactive` and `complete` promises

README.js

function onstate(readyStates, resolve) {
	function handler() {
		if (readyStates.includes(document.readyState)) {
			document.removeEventListener('readystatechange', handler);

			resolve();
		}
	}

	document.addEventListener('readystatechange', handler);

	handler();
}

document.interactive = new Promise(function (resolve) {
	onstate(['interactive', 'complete'], resolve);
});

document.complete = new Promise(function (resolve) {
	onstate(['complete'], resolve);
});