<pre id="output"></pre>

pre {
	padding: 0 1em;
	tab-size: 2;
	font: 80%/1.4 Consolas, Monaco, monospace;
}

let style = document.body.style;
let shorthands = {};
let properties = Object.getOwnPropertyNames(style.hasOwnProperty("background")? style : style.__proto__);

properties = properties.filter(p => style[p] === "") // drop functions etc
	.map(prop => { // de-camelCase
		prop = prop.replace(/[A-Z]/g, function($0) { return '-' + $0.toLowerCase() });

		if (prop.indexOf("webkit-") > -1) {
			prop = "-" + prop;
		}

		return prop;
	});

for (let p of properties) {
	style.setProperty(p, "unset");
	
	let props = [...style];
	
	if (props.length > 1) {
		// It's a shorthand!
		shorthands[p] = props;
	}
	
	style.setProperty(p, "");
}

output.textContent = JSON.stringify(shorthands, null, "\t");