What it does is to render the raw HTML, CSS, and JS file and show a preview using YQL, then process all links, frames, scripts and styles, and load each of them using YQL, so they can be evaluted in the browser.
If your hosting your Github Pages in HTML, you can also render it directly from your GitHub repositories.
HTMLView has been tested and found working with the most common web browsers, like Chrome, Firefox, IE and etc.