Есть ли встроенный HTML-валидатор в любом крупном браузере?

HTML Validator extension for Google ChromeRobert's talk - Web...

Validate a live DOM with Google Chrome and CSS HTML Validator

4 HTML Validator Extensions For Chrome

W3C (x) HTML Validator « Google Chrome Extensions

Display one of 3 icons (valid, warning(s), error(s)) in navigation bar related to validity of (x)HTML document currently shown. Errors/warnings can be displayed as page popup or with every details returned from w3c in a new tab.

HTML Validator - Интернет-магазин Chrome

HTML Validator Chrome Extension Download for Google chrome...

Is there a built-in HTML validator in any major browser?

In Firefox, there's a Extension called “Html Validator”. It adds a little indicator icon at the bottom right corner of your window. When a page you visit isn't valid, it lights up. You can click on it to see the errors.