A while ago I needed to validate some HTML on-line and found the W3C Markup Validation Service.
There are two versions of it that can take different parameters, so below are some permutations of the URLs.
But first why there are two versions, which is explained in W3C Markup Validation Service – Wikipedia
The Markup Validation Service is a validator by the World Wide Web Consortium (W3C) that allows Internet users to check pre-HTML5 HTML and XHTML documents for well-formed markup against a document type definition. Markup validation is an important step towards ensuring the technical quality of web pages. However, it is not a complete measure of web standards conformance. Though W3C validation is important for browser compatibility and site usability, it has not been confirmed what effect it has on search engine optimization.
As HTML5 has removed the use of DTD in favor of a “Living Standard”, the traditional Markup Validation Service is not applicable to these formats. Validation is instead performed using an open-source “Nu Validator”, an instance of which is provided by W3C.
The Wiert Corner - irregular stream of stuff 