Acid.JS Validator is a free bookmarklet that uses the W3C SGML parser’s API to validate the markup of the page on which it is called. To start using the bookmarklet, go to this page and follow the instructions.
To validate a page, simply click the bookmark and you will be presented with a detailed validation report. In you are a chrome user, you can get and install the bookmarklet from Google Web Store.
Two new features are already available with W3C Validator.JS – Bulgarian language version (still incomplete, but I will finish it in the next few days) available from a link in the footer and in the context menu, and option for adding validation link and image to W3C Validator.JS from other websites, so their visitors can validate the code via W3C Validator.JS.