JavaScript не меняет css.Чт делать?

<style>
	.box {
		height: 100px;
		width: 100px;
		background: red;
		margin-top: 10px;

	}

	button {
		height: 100px;
		width: 100px;
		background: green;
		margin-top: 10px;
	}


</style>

<div class="box"></div>
<div class="box"></div>

<button></button>
<button></button>

btn = document.getElementsByTagName('button'),
boxes = document.getElementsByClassName('box');

btn.style.width = "200px";