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. The really important feature of this extension is that it does not make a connection to w3c's validator. The same validating SGML parser used by w3c is bundled. This means it can validate local HTML files. (This is most the important use for me, as I do web development with manually coded html files. Each time I preview my HTML in a browser, I can also know whether it has validation errors.)
Is there anything similar in Google Chrome, Opera, Safari, or even IE? When I looked in the past years, all other validator I've seen simply send the current url to w3c's validator site.
Answer
I am the author of the Firefox extension. I have recently rewritten it for Chrome and the new Webextension API of Firefox.
See more info on the extension homepage:
The direct link from the Google Chrome store is this:
https://chrome.google.com/webstore/detail/html-validator/mpbelhhnfhfjnaehkcnnaknldmnocglk
Enjoy,