Play with the html and css all you want, I'm going to replace this page in the next lesson anyway.
As you play with it you will find strange things can happen or that nothing happens when it should have. Keep track of dots, brackets, colons and semicolons. That's often where things go wrong.
You will always have a backup in that you can just reload my copy off the server, open the source code, copy and paste into your editor (probably Notepad), and try again.
The main thing is to not get discouraged. There is a lot to know and you are just getting started.