<h> polyfill

h.css

h {
	display: block;
}

h6, h {
	font-size: 0.67em;
	margin: 2.33em 0;
}

h5, h[level="5"] {
	font-size: 0.83em;
	margin: 1.67em 0;
}

h4, h[level="4"] {
	font-size: 1em;
	margin: 1.33em 0;
}

h3, h[level="3"] {
	font-size: 1.17em;
	margin: 1em 0;
}

h2, h[level="2"] {
	font-size: 1.5em;
	margin: 0.83em 0;
}

h1, h[level="1"] {
	font-size: 2em;
	margin: 0.67em 0;
}

h.js

(function (nodeName, sectionRE) {
	function defineHeading(node) {
		// initialize heading level as 1
		var level = 1, parent = node;

		// while parent element exists
		while (parent = parent.parentElement) {
			// if parent element is a sectioning element
			if (sectionRE.test(parent.nodeName)) {
				// increase heading level
				++level;
			}
		}

		// IE9-11 DOMSubtreeModified needs a timeout
		setTimeout(function () {
			// define heading role
			node.setAttribute('role', 'heading');

			// define heading level
			node.setAttribute('level', level);
			node.setAttribute('aria-level', level);
		});
	}

	function defineHeadings() {
		Array.prototype.forEach.call(document.getElementsByTagName(nodeName), defineHeading);
	}

	// if heading is an unknown element
	if (document.createElement(nodeName) instanceof HTMLUnknownElement) {
		defineHeadings();

		document.addEventListener('DOMContentLoaded', defineHeadings);
		document.addEventListener('DOMSubtreeModified', defineHeadings);
	}
})('h', /^(article|aside|nav|section)$/i);

h.legacy.js

(function (nodeName, sectionRE) {
	function defineHeading(node) {
		// initialize heading level as 1
		var level = 1, parent = node;

		// while parent element exists
		while (parent = parent.parentElement) {
			// if parent element is a sectioning element
			if (sectionRE.test(parent.nodeName)) {
				// increase heading level
				++level;
			}
		}

		// IE9-11 DOMSubtreeModified needs a timeout
		setTimeout(function () {
			// define heading role
			node.setAttribute('role', 'heading');

			// define heading level
			node.setAttribute('level', level);
			node.setAttribute('aria-level', level);
		});
	}

	function defineHeadings() {
		for (var all = document.getElementsByTagName(nodeName), index = 0; all[index]; ++index) {
			defineHeading(all[index]);
		}
	}

	// if heading is an unknown element
	if (document.addEventListener) {
		if (document.createElement(nodeName) instanceof HTMLUnknownElement) {
			defineHeadings();

			document.addEventListener('DOMContentLoaded', defineHeadings);
			document.addEventListener('DOMSubtreeModified', defineHeadings);
		}
	}
	// or this is Internet Explorer
	else {
		document.createElement(nodeName);
		document.attachEvent('onreadystatechange', defineHeadings);
		document.attachEvent('onpropertychange', defineHeadings);
	}
})('h', /^(article|aside|nav|section)$/i);