It's a shame there's no good in-browser HTML validator (for any browser, so far as I can tell). HTML Validator used to be perfect, but Firefox started changing too rapidly for it to work consistently, and the current version only works on Windows (!).
Does anyone know of a browser/plugin combination that validates pages automatically (preferably configurable to run only on localhost)? The original HTML Validator was perfect, with a small icon in the bottom right of the screen indicating valid or invalid. Total Validator is OK, but it's not in-page, so that (for example) it's very difficult to validate pages behind a login wall.
I'm in the same boat as you. My computer at work is a Mac, and I was unable to use the HTML Validator plugin. The solution is to go to the plugin's forums - http://www.htmlpedia.org/phpBB/viewforum.php?f=11&sid=e9.... One of the guys there has been really good in taking the origin plugin and compiling it for other operating systems.
Just to be clear, Firefox doesn't have a validator built-in. It's just syntax highlighting, and it has recently been enhanced to use the same HTML5 parser as the rest of the browser. So yeah you can see some of the errors you might make in your markup but it's very far from what an actual HTML5 validator should do. It doesn't even report all parse errors (and isn't meant to).
I'm pointing this out because of a recent blog post by Mozilla dev Henri Sivonen which says: "Please do not advertise the new View Source implementation by saying that Firefox now has a validator in the View Source window."
Does anyone know of a browser/plugin combination that validates pages automatically (preferably configurable to run only on localhost)? The original HTML Validator was perfect, with a small icon in the bottom right of the screen indicating valid or invalid. Total Validator is OK, but it's not in-page, so that (for example) it's very difficult to validate pages behind a login wall.