Untitled

<!DOCTYPE HTML>
<html>

<head>
    <meta charset="utf-8">
    <title>style</title>
</head>

<body>
<ul id="elem"><p>text</p></ul>

<button class="btn">Кликни сюда</button>
<script>
    let style = document.querySelector('#elem').style;
    let button = document.querySelector('.btn');
    let css = {
        li.float: left;
    }
    button.addEventListener('click', function() {
        Object.assign(style, css)
    });
</script>
</body>

</html>