While trying to find the page you requested, the server get an "error 404". If you typed the URI in by hand, then you made a typing error, or the source you are using is incorrect. If you followed a link and got this message, then the link is invalid. Note that this does not mean that the page has moved, if the page had moved then you would have been redirected to another page (or at least you would have got another message).
If the link you followed comes from my own site (me being Ian Hickson), then please e-mail me urgently so that I can fix the problem. Thanks.
You were presumably trying to get to a page that forms part of the HTML 4.0 Test Suite. You can try using The HTML 4.0 Test Suite Index to find your page.