The extension can validate the HTML sent by the server or the HTML in the memory (after Ajax execution).
Validating html page
Make sure your codes and styles validate across the board.
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.
In simple terms, validation ensures that your website complies with the standards accepted by most web designers.
That also means that it will be accessible to more people, across more web browsers and operation systems.
Validation can be defined by many different methods, and deployed in many different ways.
Server side validation is performed by a web server, after input has been sent to the server.
Syntax errors, such as open tags, extra spaces, or forgotten quotation marks, can cause a Web page to look drastically different than the creator intended, or render correctly in one browser , but not in another.
Verifying the integrity of HTML elements manually is repetitive and tedious work; it becomes even more complicated when different sets of validation rules, such as those for cascading style sheets ( CSS ) and Extensible Markup Language ( XML ) are included in the page.
The W3 Consortium has its own online validator which you can use for free.
It may be found at: https://validator.w3.org/ A CSS validator checks your Cascading Style Sheet in the same manner.
Client side validation is performed by a web browser, before input is sent to a web server.