function injectCSS(rootNode, css) {
	// https://stackoverflow.com/q/3922139
	var expando = rootNode[EXPANDO_KEY] || (rootNode[EXPANDO_KEY] = {});
	if (!expando.containsStyles) {
		expando.containsStyles = true;
		css = '/* Chart.js */\n' + css;
		var style = document.createElement('style');
		style.setAttribute('type', 'text/css');
		style.appendChild(document.createTextNode(css));
		rootNode.appendChild(style);
	}
}
Injects CSS styles inline if the styles are not already present.
@param {HTMLDocument|ShadowRoot} rootNode - the node to contain the <style>.
@param {string} css - the CSS to be injected.