JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html>
    <head>
        <link rel="stylesheet" type="text/css" href="interface.css" />
        <title> Calculator Game </title>
    </head>
    <body>
        <div id="bgCal">
            <div id="display">
                <span id="numberTot"> 0 </span>
            </div>
            <button class="button" onclick="numberClick(1)">+</button>
            <div class="text">
                Every click increase of 1 the total number.
            </div>
            <br>
            <button class="button" onclick="buyStudent()">
                Buy Student
                <br>
                Cost: <span id="studentLUCost">10</span>
                <br>
                Students owned: <span id="studentLevel">0</span>
            </button>
            <div class="text">
                A student can click instead of yourself, but they are slower.
                <br>
                <span id="studentProd">0</span> num/sec
            </div>
        </div>
        <script type="text/javascript" src="main.js"></script>
    </body>
</html>

CSS

html {
    background-color: #e6eeff;
    font-family: Verdana;
}

* {
    border: 1px solid black;
}

#bgCal {
    background-color: #004466;
    color: white;
    width: 500px;
    margin: 0px auto;
    border-radius: 10px;
    padding: 10px 10px 10px 10px;
}

#display {
    background-color: #d7d7c1;
    color: black;
    text-align: right;
    width: 400px;
    height: 100px;
    font-size: 30px;
    margin: 10px auto;
    line-height: 100px;
    border-radius: 5px;
}

#numberTot {
    background-color: #d7d7c1;
    vertical-align: middle;
    line-height: normal;
    margin-right: 5px;
}

.button {
    background-color: #0099e6;
    width: 150px;
    height: 60px;
    border-radius: 5px;
    margin: 10px 50px;
    display: inline-block;
    text-align: center;
    overflow: hidden;
    vertical-align: middle;
}

.text {
    background-color: #004466;
    color: white;
    height: 60px;
    width: 180px;
    margin: 10px 50px 10px 0px;
    display: inline-block;
    text-align: center;
    font-size: 12px;
    overflow: hidden;
     vertical-align: middle;
}