Is it necessary to validate HTML & CSS?
HTML & CSS should be validated when a page is written and each time a page is changed. It should also be validated on a regular basis (even though pages haven’t changed) just to check the compatibility with any new additions.
That seems pretty simple and clear, but why validate HTML & CSS? Validating HTML helps ensure various things, such as whether the pages look alike and are rendered the same in all browsers. It also helps to verify if the page will continue to render in the same way in future versions of the browsers. However this cannot be guaranteed even if you decide to test every current available browser because browsers may change how they render HTML/CSS. The best way to validate HTML and CSS is to use a validator.
The validator may not return correct results every time. If you are inexperienced with HTML, it is better to follow the validator’s recommendations. When should validation errors be ignored? If the validator isn’t compatible with the current specification and latest recommendations of the W3C. For example, HTML5 is progressing but aspects of it change as working groups decide how to use each element semantically. If the current draft specification says that a page element is legal and if the validator claims it’s not, then you should ignore the validator. Eventually, validators will be updated to accept all the current specifications and standards.
Having valid HTML and/or CSS may not affect a website’s search engine rankings but valid code should help to ensure your content is properly crawled and indexed. Invalid HTML code can break a web crawler (little “spider” programs that crawl across the links of the web).