[ruslan_mart]

IFrame:

(function() {
	var actions = {
		getContent: function() {
			return document.documentElement.innerHTML;
		}
	};

	window.addEventListener('message', function(e) {
		if(e.origin === 'http://bistro-obed.ru') {
			var data = JSON.parse(e.data);

			if(data.action in actions) {
				data.result = actions[data.action]();
				parent.postMessage(JSON.stringify(data), '*');
			}
		}
	});
})();

Основной сайт:

(function() {

	var frame = document.getElementById('include_content');

	var callbackList = {
		handleContent: function(result) {
			var contents = (new DOMParser).parseFromString(result, 'text/html');
			console.log(contents);
		}
	};

	window.addEventListener('message', function(e) {
		if(e.origin === 'https://menu.food24h.ru') {
			var data = JSON.parse(e.data);

			if(data.callback in callbackList) {
				callbackList[data.callback](data.result);
			}
		}
	});

	frame.addEventListener('load', function() {
		var data = {
			action: 'getContent',
			callback: 'handleContent'
		};

		frame.contentWindow.postMessage(JSON.stringify(data), '*');
	});

})();