HTML and CSS Reference
Figure 1-2. The W3C Markup Validation Service
The W3C validator can automatically analyze your markup and display any errors it encounters so you can
correct them. It will also display validation warnings , which are simply cautions about issues you might
want to address but are not quite as severe as errors; warnings can be ignored if you have good reason to
do so, but errors are flaws that really must be fixed. When no errors are found, you'll see a joyful banner
declaring that your document is valid. A document that is valid and correctly assembled according to the
rules of the language is said to be well-formed . Other validation tools are also available—both online and
offline—that can help you check your documents.
CSS also needs to be authored in accordance with the specifications, and the W3C offers a similar CSS
Most web browsers are still able to interpret and render invalid documents, but only because they've been
designed to compensate for minor errors. Valid, well-formed documents are much more stable, and you
won't have to depend on a browser's built-in error handling to display them correctly.