For XHTML5, the XML-based validity constraints on id attributes (that
is, on IDREFs), have been lifted. And so, since XHTML5 lacks those
constraints, XXE does not prevent us from adding the following element
to an XHTML5 document:
<div id='What?' >Foo</div>
Explanation: Questionmarks are amongst the “special characters” that are
not permitted inside id attributes of XHTML1.x documents – and thus such
characters would also not occur in id attributes, as long as you were
using a validating editor such as XXE. Hence, in the good old days
before XHTML5 was introduced, the issue I am taking up in this message,
would basically never occur.
However, provided the XXE user makes use of XInclude inclusions, the
validity constraints of XHTML1.x are as relevant as ever before.
And so, given the above fragment, if, using XXE, you select it, and then
select the command 'Copy as Reference' (from the bottom submenu of the
Edit menu), nothing happens. You do not get a warning telling you that
'Copy as Reference' failed because the id attribute contained characters
which, from the XInclude point of view, are forbidden. The only way to
understand that it did not work, is when you paste the reference -
without getting any result.
So basically, in the above example, currently we get the same feedback
as if we tried to apply 'Copy as Reference' to the following element
(which lacks an id attribute):
<div>Foo</div>
That is: We do not get any feedback.
Proposal for a fix:
EITHER, let XXE display a 'failure' message based on a validity
conformance check of the IDREF, every time 'Copy as Reference' is used
without success. (This requires the presence of an id attribute.)
OR, let XXE simply display a general warning, every time 'Copy as
Reference' does not yield any result - even when the reason for the
“failure” is that the element lacks an id attribute.
That is, in the case of the 'OR' option, if the user tries to apply
'Copy as Reference' to an element which lacks the id attribute, display
the general warning. And also, display the same general warning if the
content of the id attribute fails to yield any result.
Example text of general warning: “No reference was copied, either
because there was no id attribute, or (if there was an id attribute)
because the content of the id attribute of the selected element [or, if
running with the 'advanced' XInclude option, of nearest parent element]
did not meet the validity constraints required by XInclude.”
I believe at least the 'OR' option should work also when XXE is running
with 'advanced XInclude' support enabled. In my case, I was running XXE
with the 'advanced' XInclude support active, when I failed to get a
result when copying the child element of a fragment. At first I thought
the issue was specific to the advanced XInclude support, but then I came
to understand that the issue is so fundamental that it is also present
in XXE's default XInclude support mode.
Leif Halvard Silli
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