HTML and CSS Reference
In-Depth Information
Simply put, it is quite important to aim for correct markup as a solid foundation for a
Web page and to not assume the markup is correct just because it appears to render
correctly in your favorite browser.
As shown earlier, a DTD defines the actual elements, attributes, and element relationships
that are valid in documents. Now you can take a document written in (X)HTML and then
check whether it conforms to the rules specified by the DTD used. This process of checking
whether a document conforms to the rules of the DTD is called validation .
The <!DOCTYPE> declaration allows validation software to identify the HTML DTD
being followed in a document, and verify that the document is syntactically correct—in
other words, that all tags used are part of a particular specification and are being used
correctly. An easy way to validate a document is simply to use an online service such as the
W3C Markup Validation Service, at If the malformed example
from the previous section is passed to this service, it clearly shows that the page has errors:
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