If you view the source code of a typical web page, you are likely to see something like this near the top:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
and/or
<html xmlns="http://www.w3.org/1999/xhtml" ...>
These refer to HTML DTDs and namespace documents hosted on W3C's site.
Note that these are not hyperlinks; these URIs are used for identification. This is a machine-readable way to say "this is HTML". In particular, software does not usually need to fetch these resources, and certainly does not need to fetch the same one over and over! Yet we receive a surprisingly large number of requests for such resources: up to 130 million requests per day, with periods of sustained bandwidth usage of 350Mbps, for resources that haven't changed in years.
The vast majority of these requests are from systems that are processing various types of markup (HTML, XML, XSLT, SVG) and in the process doing something like validating against a DTD or schema.
Handling all these requests costs us considerably: servers, bandwidth and human time spent analyzing traffic patterns and devising methods to limit or block excessive new request patterns. We would much rather use these assets elsewhere, for example improving the software and services needed by W3C and the Web Community.
