You can get started easily with HTML5 Addons. First, download the file ADDONS.css from the files section. Then, check your downloads folder to find the file ADDONS.css. You may change it to a location as you like
Then, copy the file to your project folder. Then, at head section of the files, link the file:
<link rel="stylesheet" type="text/css" href="ADDONS.css">
At the body section of the file, type
<centre>
</centre>
The result should be:
#Any content aligned here will be aligned in the centre.
Um.. Iam new to Github so I do not really know how to use these stuff, so the code got a bit bluffed up, right guys.