Whoops!  I replied only to Lars and not the whole group.  I don't want to
leave an incorrect impression hanging out there.  The point is that there
will be some graceful degradation in XML agents, but XHTML is still a
presentation oriented markup language.  If you try to use an XHTML document
as input to another program, a validating parser will throw an error if you
add tags.  In my mind, the true benefit of XML is the ability to write a
data source that can be used by many agents.  This is why I thought the
original article was a little misleading.

I replied to Lars:

Not to worry Lars.  From the W3C recommendation
(http://www.w3.org/TR/xhtml1/):

3.2 User Agent Conformance
    A conforming user agent must meet all of the following criteria:

    1.) In order to be consistent with the XML 1.0 Recommendation [XML], the
user agent must parse and evaluate an XHTML document   for well-formedness.
If the user agent claims to be a validating user agent, it must also
validate documents against their referenced DTDs according to [XML].

    ...

    4.) If a user agent encounters an element it does not recognize, it must
render the element's content.
    5.) If a user agent encounters an attribute it does not recognize, it
must ignore the entire attribute specification (i.e., the attribute and its
value).
    6.)If a user agent encounters an attribute value it doesn't recognize,
it must use the default attribute value.

So graceful degradation is still there.  It should still work just like the
browsers of today.  I'm guessing that you will be able to set the
conformance level on your browser.  Adding a new element would violate rule
1, but rule 4 shows there is a provision for unknown elements.  Since I have
not seen any XHTML conforming agents, I don't know how they will resolve
those two rules.  We'll see.

Mark Wonsil
4M Enterprises, Inc.


Lars Wrote:

>It was always my impression that the browsers are "forgiving" to provide
>a kind of "graceful degradation" in HTML i.e. allow older browsers still
>show some reasonable output when faced with newer HTML dialects. However,
>I haven't visited www.anybrowser.org lately, so I might be somewhat "down
>the garden path" on this HTML design feature.