<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<link rel="stylesheet" href="style.css">
<title>Set-time</title>
</head>
<body>
<input type="button" value="START" id="start">
<div class="main">
<div class="inside"></div>
</div>
<script src = "script.js"></script>
</body>
</html>"use strict"
let btn = document.getElementById("start"),
box = document.querySelector('.main .inside');
//document.box.style.backgroundColor = 'blue';
console.log(box);<div class="inside"></div>