395611 tutorial website

I am working on a tutorial website that it teaches to the user the basics of html. i have three problems. I want the users to be able to add their own code in the examples and test it. Also in some of the quizzes when i answer correct it gives me wrong answer as the answer is html code and also i want the website to be in the middle in all of the browsers. i got it in the middle on PC but on Mac I got it to the left. here is my link you can view my website [login to view URL]