Answer:
The main elements of an HTML document are the HTML element, the HEAD element, and the BODY element.
To pass the W3C specifications, you also need to have the TITLE element.
The HTML element is the container element for all the code on the page.
The HEAD element is a child of the HTML element, and must be the first child. There can only be one. This element holds a lot of "meta data" or data about the page. For instance, the TITLE element is a child of this element. As are the META tags used for things like search engine descriptions and keywords. CSS styles and JavaScript are also frequently contained herein.
The BODY element is a child of the HTML element, and must be the only BODY element in the entire page. It must come immediately following the HEAD element. The BODY element is used to contain all of the page's content. Virtually all HTML tags are descendants of the BODY.