I started using HTML Validator when I got serious about writing simple but clean HTML.
It helps me catch errors while trying to update my pages in the early morning hours when typos most occur. This extension is a must if your serious about learning HTML.
This extension will locate where errors are located in the page your visiting, just make sure you refresh the browser if you backup to a page you've visited before. It will report a false error if you go to a site that has errors and then you go back to a page that doesn't.
If your writing HTML code in my case for fun
but want the pages to look right this extension is a must. If you want to code and code right get this extension.
Thanks again Marc Gueury for making this extension happen.

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