When you visit a Github project with, for example, a demo.html page you can simply click on the "Run" button and the page will be loaded directly into your browser without the need to download the full git project in the correct directory and play with your browser to find where you downloaded it.
Instructions :
- Store the below PHP file to your web server;
- Edit the below Greasemonkey file with the URL of your webserver;
- Load the Greasemonkey script into your web browser;
- Go to any file stored on Github and you'll see a "Run" button just after the "History" button.
Once you did all the steps then you can try it.
For example go to https://github.com/prezjordan/dynamo.js/blob/master/test/test.html and you should see the Run button: