(In reply to comment #0)
> I've noticed that some pages are reporting valid at one point in time and
> invalid later despite nothing in the page having changed.

I find this surprising. One way to debug this would be to check 

(In reply to comment #1)
> I find this surprising. One way to debug this would be to check 

(In reply to comment #2)
> It was the same here until just now when I checked. At the moment, it's
> reporting as valid for me.

Ack. It's not one validator being inconsistent, it's two servers acting
differently:
http://128.30.52.13/check?uri=http%3A%2F%2Fdeeden.co.uk%2Fmisc%2Fquantum.html&debug

http://128.30.52.49/check?uri=http%3A%2F%2Fdeeden.co.uk%2Fmisc%2Fquantum.html&debug

It looks like these two servers are using different versions of the XML
libraries, but the difference in result are disturbing. Will look into that.

(In reply to comment #3)
> 
> Ack. It's not one validator being inconsistent, it's two servers acting
> differently:
> http://128.30.52.13/check?uri=http%3A%2F%2Fdeeden.co.uk%2Fmisc%2Fquantum.html&debug
> 
> http://128.30.52.49/check?uri=http%3A%2F%2Fdeeden.co.uk%2Fmisc%2Fquantum.html&debug
> 
> It looks like these two servers are using different versions of the XML
> libraries, but the difference in result are disturbing. Will look into that.

Cheers for that Olivier. I'm relieved that it's not something I did.

I am finding incompatibilities between libxml2 and XML::LibXML, two lower-lever
libraries used by the validators, but only for certain versions. This is very
puzzling, to say the least. 

Below, the script I used to test on  various machines, and a number of results.
The results saying "attributes construct error" are the proper, expected ones.

I will try and contact the maintainer(s) for XML::LibXML, in hope that they can
be of help.


#!/usr/bin/perl
use 5.008; use strict; use warnings; use utf8; use XML::LibXML qw();
my $dotted=XML::LibXML::LIBXML_DOTTED_VERSION;
print "XML::LibXML Version: $XML::LibXML::VERSION\nlibxml2 Version:
$dotted\n\n";
XML::LibXML->new()->parse_string('<foo attr1="value1"attr2="value2" />');



XML::LibXML Version: 1.66
libxml2 Version: 2.6.16

:1: parser error : attributes construct error
<foo attr1="value1"attr2="value2" />
                   ^
:1: parser error : Couldn't find end of Start Tag foo line 1
<foo attr1="value1"attr2="value2" />
                   ^
:1: parser error : Extra content at the end of the document
<foo attr1="value1"attr2="value2" />
                   ^ at testlibxml.pl line 5


**************************************************************************

XML::LibXML Version: 1.68
libxml2 Version: 2.6.16

:1: parser error : attributes construct error
<foo attr1="value1"attr2="value2" />
                   ^
:1: parser error : Couldn't find end of Start Tag foo line 1
<foo attr1="value1"attr2="value2" />
                   ^
:1: parser error : Extra content at the end of the document
<foo attr1="value1"attr2="value2" />
                   ^ at testlibxml.pl line 5



**************************************************************************

XML::LibXML Version: 1.69
libxml2 Version: 2.6.16

:1: parser error : attributes construct error
<foo attr1="value1"attr2="value2" />
                   ^
:1: parser error : Couldn't find end of Start Tag foo line 1
<foo attr1="value1"attr2="value2" />
                   ^
:1: parser error : Extra content at the end of the document
<foo attr1="value1"attr2="value2" />
                   ^ at testlibxml.pl line 5



**************************************************************************

XML::LibXML Version: 1.66
libxml2 Version: 2.6.32

:1: parser error : attributes construct error
<foo attr1="value1"attr2="value2" />
                   ^
:1: parser error : Couldn't find end of Start Tag foo line 1
<foo attr1="value1"attr2="value2" />
                   ^
:1: parser error : Extra content at the end of the document
<foo attr1="value1"attr2="value2" />
                   ^ at testlibxml.pl line 5


**************************************************************************



XML::LibXML Version: 1.69
libxml2 Version: 2.6.32

:1: parser error : Extra content at the end of the document


**************************************************************************

XML::LibXML Version: 1.63
libxml2 Version: 2.6.29

:1: parser error : attributes construct error
<foo attr1="value1"attr2="value2" />
                   ^
:1: parser error : Couldn't find end of Start Tag foo line 1
<foo attr1="value1"attr2="value2" />
                   ^
:1: parser error : Extra content at the end of the document
<foo attr1="value1"attr2="value2" />
                   ^ at testlibxml.pl line 5


**************************************************************************

XML::LibXML Version: 1.68
libxml2 Version: 2.6.27

:1: parser error : Extra content at the end of the document



**************************************************************************

(In reply to comment #4)
> Cheers for that Olivier. I'm relieved that it's not something I did.

No problem Steve, and many thanks for the report.

For the time being I have downgraded the version of the XML::LibXML library
(now using 1.66 which seems to work better) and the two validator.w3.org
servers are producing the proper (and consistent) output.

Will keep thi sbug open until we have a satisfying resolution of the library
problem, not just this workaround.

I think I found the culprit in the Changelog for XML::LibXML. In recent
versions, there is a new module to use the Structured Errors API (great!) but
it's not quite backward compatible.

Will have to add code to handle the Structured Errors. 
http://search.cpan.org/~pajas/XML-LibXML-1.69/lib/XML/LibXML/Error.pod

(In reply to comment #8)
> I think I found the culprit in the Changelog for XML::LibXML. In recent
> versions, there is a new module to use the Structured Errors API (great!) but
> it's not quite backward compatible.
> 
> Will have to add code to handle the Structured Errors. 
> http://search.cpan.org/~pajas/XML-LibXML-1.69/lib/XML/LibXML/Error.pod

The frustrating part so far is that the new structured errors code only gives
you the last parsing error (when, if anything, I would be happy enough showing
only the first!)

Using this code, from the perl module documentation:

use 5.008; use strict; use warnings; use utf8; use XML::LibXML qw();
my $dotted=XML::LibXML::LIBXML_DOTTED_VERSION;
print "XML::LibXML Version: $XML::LibXML::VERSION\nlibxml2 Version:
$dotted\n\n";

eval {XML::LibXML->new()->parse_string('<foo attr1="value1"attr2="value2"
/>')};
if (ref($@)) {
  # handle a structured error (XML::LibXML::Error object)
  print $@->dump();
} elsif ($@) {
  # error, but not an XML::LibXML::Error object
  print $@;
} else {
  # no error
}


ot@qa:~$ perl testlibxml.pl 
XML::LibXML Version: 1.66
libxml2 Version: 2.6.32

:1: parser error : attributes construct error
<foo attr1="value1"attr2="value2" />
                   ^
:1: parser error : Couldn't find end of Start Tag foo line 1
<foo attr1="value1"attr2="value2" />
                   ^
:1: parser error : Extra content at the end of the document
<foo attr1="value1"attr2="value2" />
                   ^ at testlibxml.pl line 6


ot@qa:~$ perl testlibxml.pl 
XML::LibXML Version: 1.69
libxml2 Version: 2.6.32

$error = bless( {
                  'num1' => 0,
                  'file' => '',
                  'message' => 'Extra content at the end of the document
',
                  'domain' => 1,
                  'level' => 3,
                  'str2' => undef,
                  '_prev' => undef,
                  'str1' => undef,
                  'str3' => undef,
                  'num2' => 11,
                  'code' => 5,
                  'line' => 1
                }, 'XML::LibXML::Error' );

Maybe I'm doing it wrong? The documentation is scarce, might need to contact
the developer(s) to get some clearer answers.

Followup archived here:
http://lists.w3.org/Archives/Public/public-qa-dev/2009Jan/0003.html

Hoping Petr will have time to respond.

Got a prompt reply from Petr:
http://lists.w3.org/Archives/Public/public-qa-dev/2009Jan/0004.html
and now waiting for the release of the version 1.70 of XML::LibXML
http://search.cpan.org/dist/XML-LibXML/

Things are going to be tricky then, because any system with a version of
XML::LibXML between 1.67 and 1.69 (inclusive) will have slightly wrong error
reporting for xml-wf issues. I'm wondering whether this would be acceptable, or
whether to require >= 1.70, which may then be a burden.

To be continued...

*** Bug 4420 has been marked as a duplicate of this bug. ***

Ah-ha! 

XML-LibXML-1.69_1 developer release. Should be enough to start work on
implementing the structured errors in the validator.
http://cpansearch.perl.org/src/PAJAS/XML-LibXML-1.69_1/Changes

Implementation done, ready for next release:
http://lists.w3.org/Archives/Public/www-validator-cvs/2009Feb/0092.html
and
http://lists.w3.org/Archives/Public/www-validator-cvs/2009Feb/0136.html

Note that the code above does rely on a developer version of XML::LibXML -
might be a bit of trouble for people with their own instance. But with time,
we'll be fine.

Here is a one liner to text which version of XML::LibXML your system has:

perl -MXML::LibXML -e 'print " XML::LibXML Version: $XML::LibXML::VERSION\n";' 


If you have a version > 1.66 and < 1.70, I suggest heading to CPAN and install
the latest version:
http://search.cpan.org/dist/XML-LibXML/