Text that isn’t processed by the user agent (browser) can be beneficial for a couple of reasons. The first reason is simple documentation, that is, to make your documents more legible and easy to follow should you need to edit them later. The second is to include more information within the document for later access or inclusion by features supported in upcoming browsers.
HTML comments can be used by the developer of the web pages to document certain sections in the web page. A comment in HTML should be written as follows:
<!– Your comment here. –>
Comments are generally used for short text, not for commenting out large sections of HTML code. The text included in the comments is not visible to the user.
For larger comment sections, CDATA (character data) sections can be used. CDATA structures are also useful in XML and XHTML. The format for the CDATA tag is as follows:
<![CDATA[ Commented text goes here. ]]>
Both comments and CDATA tags cannot be nested.
The term CDATA is used about text data that should not be parsed by the XML parser. Characters like “<” and “&” are illegal in XML elements. “<” will generate an error because the parser interprets it as the start of a new element. “&” will generate an error because the parser interprets it as the start of a character entity.