This article discusses what validation means, points you to some of the free tools that you can use, and deals with its limitations and the problems that a new webmaster may face. Otherwise you'll be completely lost here since I assume you at least know what these terms mean.Note: if you are not sure what HTML and CSS mean, please read What are HTML, CSS, Java Script, PHP and Perl? For those unfamiliar with the term, "validating" a page is just a jargon-filled way of referring to the use of a computer program to check that a web page is free of errors.That is, it will check that it complies with the CSS standards set by the W3 Consortium.There are a few which will also tell you which CSS features are supported by which browsers (since not all browsers are equal in their CSS implementation).
by Christopher Heng, Whether you design your web page using a visual web editor like Expression Web, Dreamweaver or Blue Griffon, or you code HTML directly with a simple text editor, the generally recommended practice is to validate it after you finish designing it.
The developer can use online or off-line validators (including validation tools built into editors) to check the validity of the XML documents.
The xmlvalidate task of Apache Ant can be used for batch validation of XML files.
The Applicability section explains the scope of the technique, and the presence of techniques for a specific technology does not imply that the technology can be used in all situations to create content that meets WCAG 2.1.
The objective of this technique is to avoid ambiguities in Web pages that often result from code that does not validate against formal specifications.