From MichaelCurrin/jekyll-blog-demo:

---
layout: default
permalink: /404.html
---

<style type="text/css" media="screen">
    .container {
        margin: 10px auto;
        max-width: 600px;
        text-align: center;
    }

    .container h1 {
        margin: 30px 0;
        font-size: 4em;
        line-height: 1;
        letter-spacing: -1px;
    }

</style>

<div class="container">
    <h1>404</h1>

    <p><strong>Page not found :(</strong></p>
    <p>The requested page could not be found.</p>
</div>