1. All elements are well-formed
2. A single element, known as the root element, contains all of the other elements in the document

1. Opened and subsequently closed
2. An empty element

<i><b>word</i>

<i><b>word</i></b>
                        

1. Documents may not be well-formed
2. Element and attribute names may not be in lower case.
3. For non-empty elements, end tags are not always required.
4. Attribute values may not always be quoted.
5. Entity references as hex values may not only be in lowercase