See the link to the w3c - several times they've started delivering 503 HTTP error codes, hoping that the applications would start to break. It didn't have a big effect, either because the application didn't actually use the DTD they've retrieved for anything or because they broke in a non-obvious manner (like with the servers I'm administering). Had the outage been shorter or if I wasn't monitoring the Tomcat JVMs it could've stayed under the radar. That's one of the reasons I've made this submission.