New "end of document in prolog" error

Starting today, all my pages which used to validate correctly are now
giving this error message:

Error Line 1, Column 1: end of document in prolog
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
This error may appear when the validator receives an empty document.
Please make sure that the document you are uploading is not empty, and
report any discrepancy.

The pages haven't changed, so the validator must have.

A browser still displays the pages correctly, so I am certain they are
not empty.

     - Rich

Received on Wednesday, 6 September 2017 09:14:25 UTC