Title anchors

title-anchors.css

.anchor {
    float: left;
    padding-right: 4px;
    margin-left: -20px;
    display: none;
}

h1:hover .anchor, h2:hover .anchor, h3:hover .anchor {
    display: inline;
}

title-anchors.php

function anchor_content_h($content, $level) {
    $pattern = '#<h' . $level . '>(.*?)</h' . $level . '>#';
    $content = preg_replace_callback($pattern, function ($matches) use ($level) {
        $title = $matches[1];
        $slug = sanitize_title_with_dashes($title);
        return '<h' . $level . '>'
            . '<a id="' . $slug . '" class="anchor" href="#' . $slug . '" aria-hidden="true"><svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>'
            . $title
            . '</h' . $level . '>';
    }, $content);
    return $content;
}