Getting Started - HTML5 with CSS3 - page 13

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
validation service ( http://jigsaw.w3.org/css-validator/ ) to check your CSS files for problems.
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.