Untitled

<!DOCTYPE HTML>
<html>
<head>
    <meta charset="utf-8">
    <title>length</title>
</head>
<body>

<div id="elem"><p>text</p></div>

<button class="btn><del>Кликни сюда</del></button>
<button class="btn1"><b>Кликни сюда</b></button>
<button class="btn2"><font color="red">Кликни сюда</button>

<script>
    let p = document.querySelector('#elem');
    let button = document.querySelector('.btn');
    let button1 = document.querySelector('.btn1');
    let button2 = document.querySelector('.btn2');

    button.addEventListener('click', function() {
        elem.style = 'del';
    });
    button1.addEventListener('click', function() {
        elem.style = 'b';
    });
    button2.addEventListener('click', function() {
        elem.style.Color   = 'red';
    });
</script>
</body>
</html>