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Is there a plugin for firefox to validate HTML 5? I've always used this but if my webpage has a <!DOCTYPE HTML> doctype it complains about it not having a doctype at all.

I specifically would like one similar to the one I linked, where the verification happens on your computer and instantaneous and automatic, without needing to send any data to a 3rd party server and then wait for a reply.

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This is the best one I found so far:

https://github.com/rogerjohansson/html5validator (see Downloads for an xpi)

You should not use the validator.nu online service with it, since it causes a DoS attack on the online service, and you will probably be banned from using the service as a countermeasure (see https://github.com/rogerjohansson/html5validator/issues/6). The cool thing is, it does not send the URL to the validator, but the HTML data directly; this means that local sites or password-protected sites can also be checked.

Validation can be turned on automatically, by a domain-whitelist or by clicking the validator item.

You can and should up your own validator.nu-instance (at least on Linux and Mac OS X) - see this. You would then have a locally-running validator and can even check sites without an Internet connection by filling in http://localhost:8888/ (the default address of the local validator instance) into the Validator URL of the addon.

To give credits: I have the information above from Validating HTML5 with validator.nu and the HTML5Validator Extension for Firefox.

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On the Web Developer add-on, https://addons.mozilla.org/en-US/firefox/addon/60, if you use "Tools" > "Display Page Validation" it will show a bar at the top validating the page source code. This is done via the W3C validator, but without you having to navigate there.

It is a shame the HTML Validator has no plans as of yet to incorporate HTML5 until at least version 0.9.

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This says it will do it: https://addons.mozilla.org/en-US/firefox/addon/2318 and I found this also: http://html5.validator.nu/

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1  
The problem with the first is that it requires me to click the icon to validate. I want it to automatically validate every page I visit so that I realize immediately if my website doesn't validate, like HTML Validator does. The second is web-based, there is an "official" one at the W3C website too. – Andreas Bonini Feb 10 '10 at 19:36

You can validate HTML5 at the following site:

http://validator.w3.org/

Make sure on the options that you select the HTML5 document type.

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