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- XHTML elements must be properly nested
- XHTML elements must always be closed
- XHTML elements must be in lowercase
- XHTML documents must have one root element
XHTML Elements Must Be Properly Nested
In HTML, some elements can be improperly nested within each other, like this: This text is bold and italic In XHTML, all elements must be properly nested within each other, like this:
This text is bold and italic
A common mistake with nested lists, is to forget that the inside list must be within and tags. This is wrong:
This is correct:
Notice that we have inserted a tag after the tag in the "correct" code example.
XHTML Elements Must Always Be Closed Non-empty elements must have an end tag.
This is wrong:
This is a paragraph
This is another paragraph This is correct:
This is a paragraph
This is another paragraph Empty Elements Must Also Be Closed
Empty elements must either have an end tag or the start tag must end with />. This is wrong:
A break:
A horizontal rule:
An image:  This is correct:
A break:
A horizontal rule:
An image: XHTML Elements Must Be In Lower Case
The XHTML specification defines that the tag names and attributes need to be lower case. This is wrong:
This is a paragraph
This is correct:
This is a paragraph
XHTML Documents Must Have One Root Element
All XHTML elements must be nested within the root element. All other elements can have sub (children) elements. Sub elements must be in pairs and correctly nested within their parent element. The basic document structure is:
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