W3C Link Checker fails with an Error: 406 Not Acceptable

Attempting to run the W3C Link Checker against //ardamis.com/ returns an error message.

Error: 406 Not Acceptable

This is what the W3C says about the 406 HTTP status header:

406 Not Acceptable
The resource identified by the request is only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request.


In other words, the W3C Link Checker requests the web page, and tells the web server that, by the way, it can only accept a responses in a certain format. The web server then regrets to inform the requestor that it cannot fulfill this request, because it cannot return a response that would be acceptable to the requestor. It does this in the form of a 406 Not Acceptable HTTP header. The W3C Link Checker then outputs this error.

Other W3C apps, like Unicorn – W3C’s Unified Validator and the W3C HTML Validator don’t seem to be sending the same HTTP headers. (But I did note that there were a few small issues preventing the home page from passing the test, which I then fixed.)

Ardamis runs on WordPress, with a custom theme originally developed years ago from the Kubrick theme and a handful of plugins (as more completely described at the colophon page). I tinker with the site, from time to time, trying to speed it up or what-have-you. But no amount of tinkering seemed to resolve this problem. Over the course of a few months, I’d try various changes to the site to see if there was something I could do to fix this problem. I had pretty much convinced myself that it was going to be an issue for my web host when, miraculously, after making some changes to the .htaccess file, my theme and disabling one of the plugins (which I can’t see how would possibly affect the HTTP headers) the Link Checker began working.

In the results page for www.ardamis.com, it lists some of the headers used:

Settings used:

  • Accept: text/html, application/xhtml+xml;q=0.9, application/vnd.wap.xhtml+xml;q=0.6, */*;q=0.5
  • Accept-Language: en-US,en;q=0.8
  • Referer: sending
  • Sleeping 1 second between requests to each server

I’m not sure what I did to make this work, or even if it was something I did. But further troubleshooting would have involved disabling all plugins, trying a different theme, and then ruling out WordPress entirely.