index.js

(function() {
    var wrapper,
        html,
        css;

    /**
     * Make some HTML.
     */
    html = window.marked(document.body.innerText);

    /**
     * We better make a wrapper element for the html.
     */
    wrapper = document.createElement('div');
    wrapper.className = 'markdown-body';
    wrapper.innerHTML = html;

    /**
     * Some styling of the body element and appending
     * of the markdown wrapper.
     */
    document.body.innerHTML = '';
    document.body.style.maxWidth = '790px';
    document.body.style.margin = '0 auto';
    document.body.style.padding = '8px 8px 32px 8px';
    document.body.appendChild(wrapper);

    /**
     * Make sure to add the stylesheet link to the DOM.
     */
    css = document.createElement('link');
    css.rel = 'stylesheet';
    css.href = chrome.extension
        .getURL('components/github-markdown-css/github-markdown.css');
    document.head.appendChild(css);
}());