index.html

<style>
    *::before,
    *::after {
        display: block;
        padding: 1rem;
    }

    html {
        font-family: monospace;
        font-size: 1.25rem;
        text-align: center;
        padding: 1rem;
    }

    head {
        display: block;
    }

    body {
        display: inline;
        margin: 0;
    }

    html::before {
        content: 'This is a working website with only one HTML tag.';
    }

    html::after {
        content: 'Created by Atanas Atanasov <github.com/scriptex>.';
    }

    head::before {
        content: 'There are six text elements on this page.';
    }

    head::after {
        content: 'This is just a proof that you don\'t need HTML to code websites.';
    }

    body::before {
        content: 'It\'s a completely responsive webpage.';
    }

    body::after {
        content: 'Use CMD + Alt + U (or CTRL + U) to see the source of the page.';
    }
</style>