A validation is the validity verification of a website, for example, against applicable standards. A validator is a program that can be used to check compliance with a standard.
HTML documents are displayed correctly by many Browsers even if they are not valid. Therefore, small errors in the syntax are often unnoticed if a validator is not used specifically. Such errors can be:
Even if these errors seem unimportant at first glance, it is the job of a Webmaster to create error-free (HTML and CSS) code, as is the case in other languages. A correct syntax should be self-evident in programming and in the verbal language. Validation can also serve as a learning effect, so that the programmer does not get used to the wrong syntax.
The W3C provides its own web page  to validate documents. By entering a URL in the HTML Validator, you can check the Webpage for compliance with standards in a few seconds. The file does not have to be online for this - the settings also allow checking local files on your own computer. In addition, not only HTML documents, but also XHTML, SMIL, MathML, etc., can be validated. Applicable websites are offered for CSS and other similar documents as well.
Alternatively, there is an add-on from Firefox. It displays the errors of a page directly in the browser and may be the faster method.
The validation of a web page has two sides. The influence of validity on the ranking is controversial. Google’s own websites are not valid. The background is explained in a video . One idea is that with a large website such as Google, each byte counts during the transfer. By deliberately omitting some tags, some bytes can be saved. This reason, why most pages on the Internet are not valid, should be considered critically.
Validation means that the page should be displayed correctly on different browsers. That is the benefit of a valid page, but it is nevertheless helpful to test a newly created page in different browsers.