First you have to open the DevTools and click on the Validator. This is one more click than before. And you can't see immediately that there are problems while surfing, because the icon is always grey in the toolbar. This is unfortunately a step backwards, therefore only 3 stars instead of 5 as before.

Unable to properly parse the document, just use https://validator.w3.org

Validating frames is no longer possible with the new version when using Firefox. Hopefully, the feature will be recovered, otherwise this expansion will be of no use.

With FF 60 still sees the source generated by browser instead source generated by server. The Options of this extension are partial broken... not able to select GSML, always gets TIDY... partial working on FF 60, needs to be reworked again to be useful under FF 60 and more...

EDIT: in chrome this extension don't work anymore... won't validate my web pages, instead to see a green flag I can see only a message "Info 0 0 No HTML" and nothing other... the flag remains gray...

This happens on latest version of Chrome! Version 66.0.3359.181...

Now working again. (A great tool I have used for many years.) But
first I had to do a clean install of Firefox 57 (new profile) and then
install HTML Validator. Apparently there was some junk in my VERY old
profile that caused a problem with the new Firefox and new HTML
Validator. The source displayed by the Inspector showed a wtx-context
added to the HTML tag and to several others. That was not in my actual
source code. The Validator did not like that attribute and put up a
warning. Even worse, the Validator would not detect any errors in an
intentionally buggy test file I created. Doing the clean install fixed
all that.

I wish there were a keyboard shortcut to invoke the HTML Validator in
this new environment, e.g., as Ctrl-Shift-C, Ctrl-Shift-S, etc. do for
other tools. I have programmed my mouse to do a Ctrl-Shift-I, but then
have to click on the HTML Validator tab.

When I try to go to http://users.skynet.be/mgueury/mozilla, I get a
timeout. Is that site no longer available? If I do a tracert for
users.skynet.be, it steps thru almost 20 nodes, but finally times out in
Belgium. Jeff

very good

Ok

Excellent tool that I have used for years to check all my web pages before releasing. Please make WebExtensions compatible so it stays working after November 2017. Thank you.
Kevin (for some reason it is showing as anonymous)

It's still a great and useful extension.
Especially the activation just for certain domains and the integration into the sourcecode window are nice.
But I am afraid the addon will stop working soon, when e10 (multiprocess) support will be required.

/edit
Answer to your reply:
I have e10 disabled anyway so far. But the mozilla compatibility reporter says html validator doesn't work with multiprocess. I also checked that in a new profile (see screenshot) where e10 actually was enabled.
http://fs5.directupload.net/images/170412/jnxml2v5.png
So it may be just a problem with the automatic compability recognition or formalities in the manifest.

Also see https://www.arewee10syet.com/ for further links/info.

Good extension.

I habe 64-bit firefox also installed but i got a error in the 64-bit version:

The dynamic C library contained in the extension file could not be found

Hope this could be solved.
In 32-bit version FF everything ok.

J'ai un problème de conflit entre firebug et html validator avec les dernières versions.
Lorsque html validator est actif, l'affichage de firebug est modifié et certaines fonctionnalités ne sont plus disponibles ?

Some errors are a bit missleading when localized. Requires more help messages for errors. Or at least show english help if localized is missing.

Sadly this still doesn't work properly with CSE HTML Validator.

I've been testing HTML Validator 0.9.7.4 Beta with no problems on the version of SeaMonkey mentioned below, comparing "Tidy" results with "SGML" results. In particular the "Tidy" version found attribute errors (including quite a number of mismatches due to typos between the name= and id= attributes of tags). :-)

User agent: Mozilla/5.0 (X11; Linux x86_64; rv:50.0) Gecko/20100101 Firefox/50.0 SeaMonkey/2.47a1
Build identifier: 20160625075045
http://hg.mozilla.org/mozilla-central/rev/0e073f5ca38a002d43e92016ee40d686da4a0534
http://hg.mozilla.org/comm-central/rev/ab9359275942

Great that it has ceased being "Windows-only".

P.S. One unclosed tag caused several "anchor already defined" warnings, but once I had noticed and fixed the error the spurious messages disappeared.

When I open source code in new window then "Wrap Long Lines" option in View menu doesn't work correctly. When I switching to viewing source code in Tab then all works fine. Please fix this.

I just wanted to express my gratitude to Marc for keeping this add-on alive and updated since so many years. It's an indispensable tool that I use every day.

Hello Marc, we daily use the HTML validator extension. We are happy that it already can validate HTML5 code locally. However since couple days we are experiencing problems. If we press Ctrl+U, nothing will show up, only a white screen. If we want to see the source code at least, we must uninstall the addon first. Are you aware of this trouble? Thanks for help!

Make sure to set HtmlTidy as the validation template everyone!
The default one makes no sense.

After downloading I was presented with a table with options I could barely read - white text against a light blue background. So I dismissed the box and the user guide page was opened in a new browser tab, explaining the options - which I can setup via preferences in the extension window. I haven't got round to using the tool yet - but to be presented with options that weren't readable is off-putting! I'm not sure if this is specific to another extension I have enabled - e.g. no squint. But this was set to show black text against a white background as default. I disabled nosquint - but the html validator colors remained unreadable. So, assuming the colors have nothing to do with my existing extensions (even if they do?) - please could these choices be presented in a more readable way. Note: I don't have an array of extensions installed - nosquint, noscript, and newscrollbars only. So, seems unlikely they are to blame.

Using Linux64bit Firefox, 0.9.6.3 didn't show up in the "View Page Source" tab, so I changed FF's behavior to open source in a new window (Set "view_source.tab" to "false" in about:config). Works again.

The new version 0.9.6.3 its not only signed yet - it solved the issues with windows x64 systems on Firefox 42. I'm very happy that i can use it again. Thanks Marc!