Doctypes tell your browser what kind of markup language your pages use.
There are many different Doctypes, and with each DTD there are tags that can’t be used, which would make your web pages invalid if they are used.
I won’t list the Doctypes here (there are too many), so have a look at W3schools to see the list.
![:]](http://jammy.party-truffle.net/wordpress/wp-includes/images/smilies/kao_regular.gif)
And that’s okay, I’m trying to think of something too 
A tutorial, uhm? I should take some time to think about...