XHTML 1.0 crib sheet

4. Entities

Some characters are not displayed by XHTML user agents, either because they have a special meaning in XHTML (e.g. square brackets, "<" e ">", which enclose tag names) or because they don't belong to the character set of the document, e.g. greek or mathematical symbols.

Therefore, these characters must be represented through entities, which allow you to display special characters by using any of the following special sequences: