W3C Validator Bookmarklet
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, drag the link below to the bookmarks bar of your browser. To validate a page, simply click the bookmark. If you are Google Chrome user, you can get and install the validation bookmarklet from Google Web Store.