Update the visible-but-unfocused tab's page, styles, and scripts without ever pressing refresh.

updateElements.js

(function (document) {
	function forEach(array, callback) {
		for (var i = 0, l = array.length; i < l; ++i) {
			callback(array[i]);
		}
	}

	function refreshElements() {
		// prevent refreshing if the document is hidden or the page is focused
		if (document.hidden || document.mozHidden || document.msHidden || document.oHidden || document.webkitHidden || (document.hasFocus && document.hasFocus())) return;

		forEach(document.querySelectorAll("html,link[rel],script[src]"), function (element) {
			// prevent cross-domain requests
			if (String(element.href || element.src || location).indexOf(location.protocol + "//" + location.host) != 0) return;

			var xhr = new XMLHttpRequest;

			xhr.onreadystatechange = function () {
				if (xhr.readyState > 1) {
					xhr.lastModified = xhr.lastModified || +new Date(xhr.getResponseHeader("Last-Modified"));

					element.dataset.lastModified = element.dataset.lastModified || xhr.lastModified;

					// if the source is newer than the last request
					if (xhr.lastModified > element.dataset.lastModified) {
						if (xhr.readyState == 4 && xhr.status == 200) {
							// if refreshing the document
							if (element.nodeName == "HTML") {
								document.write(xhr.responseText);
								document.close();

								return;
							}

							// if refreshing an element
							element.dataset.lastModified = xhr.lastModified;

							// clone the element
							for (var clone = document.createElement(element.nodeName), i = 0, attr; attr = element.attributes[i]; ++i) {
								clone.setAttribute(attr.name, attr.value);
							}

							element.parentNode.replaceChild(clone, element);

							element = clone;
						}
					} else {
						xhr.abort();
					}
				}
			};

			xhr.open("GET", element.href || element.src || location, true);
			xhr.setRequestHeader("Cache-Control", "no-cache");
			xhr.setRequestHeader("Pragma", "no-cache");
			xhr.send(null);
		});
	}

	// poll the document at 5 frames/second
	setInterval(refreshElements, 200);
})(document);