header.php

<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        li{
            list-style-type: none;
            text-align: center;
        }
        #logo{
            /* float: left; */
            height: 85px;
            float: left;
        }
        #loginbtn{
            /* display: flex; */
            float: right;
        }
    </style>
</head>
<body>
    <ul>
        <li>
            <a href="">
                <img src="images/brain_logo2.png" id="logo">
            </a>
        </li>
        <li>
            <input type="button" value="Login" id="loginbtn">
        </li>
    </ul>
    <div style="clear:both;"></div>
    
</body>
</html>