What's different between HTML and WML/WAP?

I checked the source of a few WAP sites,

but doesn't find anything different from a normal HTML page.

Can you name a few detailed points?

Well, WAP/WML is very strict when it comes to markup because the page needs to be compiled before delivery to the client device.

As for specifics,

• WAP "pages" can have more than one "card". (Confusing? I know...)
• Although not markup related, accepted image formats are more limited
• Do not forget a DOCTYPE!
• Content must be served with the text/vnd.wap.wml MIME type