Editing GitHub Markdown Render

<!DOCTYPE html>
<html>

<head>
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <meta charset="utf-8" />
    <title>GitHub Markdown Render</title>

    <link rel="stylesheet" href="https://netnr.eu.org/github-markdown-css@5.5.1/github-markdown.css" />
</head>

<body>
    <textarea rows="10" placeholder="markdown content"></textarea>
    <button class="nr-btn">Render</button>
    <hr />
    <div class="markdown-body"></div>
</body>

</html>

* {
    box-sizing: border-box;
}

textarea {
    width: 100%;
}

var btn = document.querySelector('.nr-btn');
btn.addEventListener('click', function () {
    var content = document.querySelector('textarea').value;
    btn.innerHTML = "Processing";
    btn.disabled = true;

    fetch('https://api.github.com/markdown', {
        method: 'post',
        body: JSON.stringify({
            text: content,
            mode: 'gfm'
        })
    }).then(resp => resp.text()).then(res => {
        document.querySelector('.markdown-body').innerHTML = res;
    }).finally(() => {
        btn.innerHTML = "Render";
        btn.disabled = false;
    })
})