There is one: http://users.skynet.be/mgueury/mozilla/
That tool is based on HTML tidy, which is not a true DTD validator and therefore cannot detect all invalid code.
For example, current versions of HTML Tidy will not detect that the <thead>, <tfoot> and <tbody> elements inside a <table> element occur in the wrong order, because HTML Tidy has not been programmed to detect this problem. HTML Tidy fails to fix some other problems. For example, a <map> element cannot contain both an <area> element and a <form> element, but HTML Tidy is not programmed to detect this and the error will go unnoticed. A <table> element may not contain both <col> and <colgroup> elements, nor may it contain both <tr> and <tbody> elements. HTML Tidy ignores these problems too. A <table> must always contain at least one <tbody> or <tr> element, but this is not enforced by HTML Tidy when the <table> also contains other elements.