OK ish I suppose Rated 2 out of 5 stars
Firstly, the description above is confused to say the least....
"available on Windows only"
"platforms supported, OS X"
"Just a reminder, this is NOT available on mac OSX"
.....make up your mind mate ;)
Seriously though, this simply means you have to go to the website to install it if you're not a windows user, but then the issue arises that it will NEVER auto-update via the firefox interface.
Secondly, this app has a PPC version, but one that hasn't been updated FOR 3 YEARS. I'm forced to use version 0.7.9.5.
Maybe this is why this validator seems to miss more errors than MOST other firefox add-ons available?
It also has an issue with iFrames that other validators don't have.
Finally, W3C online is much more comprehensive (leading me to believe that all this needs is an up-to-date PPC build or a OS X UB build to bring it back up to spec?).
Either way, currently "Total Validator" is better at detecting issues (and conforms to W3C properly), whereas this one has the better GUI and doesn't send the html data to another site.
overall I'd give it 5/10 as it stands.
It is freeware
If you want a PPC version of the 0.841 validator, a Mac OSX PPC user will have to compile it.
There are compilation instructions here:
Unhappily, all my Mac buddy are on Intel now and are not interested in compiling a PPC version.