SOURCE

function myfunction() {
	let y = document.getElementById("year").value;
	if (y % 100 != 0 && y % 4 == 0 || y % 400 == 0) {
		document.getElementById("result").innerHTML = "YES!";
	} else {
		document.getElementById("result").innerHTML = "NO!";
	}	
}

<input type="number" id="year">
<button onclick="myfunction()">确定</button>
<p id="result"></p>