A validator is a computer program that can check whether the HTML code used on a web page is valid or not, and if it is not, provides information about where the errors are.
Validating web documents is an important step that can dramatically help improve and ensure their quality. It checks the syntax, that is, the correct closing and use of tags. This tool is used for so-called page validation and can validate most language versions from HTML 2.0 to HTML 5 or XHTML 1.1.
A validator lets you validate by entering the address of a web page directly, by typing the code directly into the validation field, or by uploading the file with the code itself.
Today there are also validators for CSS, which check the correct properties and their attributes, or their syntax.