w3c
Not possible to validate styles for error pages
I find it impossible to validate styles on error pages (e.g. HTTP 404). There is no such option in the web interface and I do not find such parameter in the documentation. I would be very nice to have this feature.
P.S. The Markup Validation Service supports such validation with option "Validate error pages" and parameter No200, but redirects to The Nu Html Checker.
If this is still an issue that isn't being fixed, one workaround could be to use a tool like wget to download the page, then validate that. You might not want to "Save" the page from your browser, as browsers often significantly alter the contents of the page being saved, whereas a downloader like wget doesn't alter the HTML at all.
