The World Wide Web Consortium sets the standards and also hosts a variety of web page validators.
In simple terms, validation ensures that your website complies with the standards accepted by most web designers.
That means they have to meet the "strict" standards set by the W3C Organization and pass a variety of validations for CSS and XHTML. Some only check CSS, others XHTML, and others for accessibility.
If you are sincere in presenting standardized pages to the public, test them with several validators.
This tool checks single HTML pages for conformance with accessibility standards to ensure the content can be accessed by everyone.
Validation is important, and will ensure that your web pages are interpreted in the same way (the way you want it) by various machines, such as search engines, as well as users and visitors to your webpage.
Using client side Java Script is an efficient way to validate the user input in web forms.
When there are many fields in the form, the Java Script validation becomes too complex.
Conforming to standards and regulations is one of the many ways you can make your website universally understood.
Make sure your codes and styles validate across the board.