<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <link href="aaa.css" rel="stylesheet">

</head>
  
<body>
    <div class="wrap">
        <section class="left"></section>
        <section class="right"></section>
    </div>

</body>

</html>
.wrap {
    width: 800px;
    margin: 100px auto;
    border: 5px solid black;
}

.wrap .left {
    width: 400px;
    height: 400px;
    background-color: red;
    float : left;
} 

.wrap .right {
    width: 410px;
    height: 400px;
    background-color: blue;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.