HTML validators operate by comparing the mark-up on a web page to the W3C standards. The standards vary depending upon the declared version and so the validator will start by reading the DOCTYPE declaration to see which set of standards to apply.
Once the validator has read the page and determined the applicable standards it looks for such things as missing opening or closing tags, missing quotation marks and other hand-coding errors.
The validator then provides a report indicating that the coding is correct or not. Errors are noted in a list. One error, such as neglecting to close a tag, can cause a cascade of errors through the page, producing dozens or even hundreds of noted errors. However when the page author addresses the first error listed it will also eliminate the "cascade errors".
Many major browsers are often tolerant of certain types of error, and may display a document successfully even if it is not syntactically correct. Certain other XML documents can also be validated if they refer to an internal or external DTD.
While the W3C and other HTML and XHTML validators will assess pages coded in those formats, a separate validator is needed to confirm that there are no errors in the associated Cascading Style Sheet. CSS validators work in a similar manner to HTML and XHTML validators in that they apply current CSS standards to referenced CSS documents.
