Error 403 accessing to SSML specification synthesis.dtd

Hi, I am developing an application that uses the SSML specification. In my
SSML files, I make a reference to the file synthesis.dtd at
http://www.w3.org/TR/speech-synthesis/synthesis.dtd. I did notice that with
Microsoft Internet Explorer (v8 and v9, don't know other versions) it is
impossible to access, receceiving a nice error '403'. No problems with
others browers like Mozilla Firefox or Google Chrome.
Any ideas?
Thanks.