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Comments about the 2002-08-05 XHTML 2.0 WD

From: Henri Sivonen <hsivonen@iki.fi>
Date: Mon, 19 Aug 2002 19:27:20 +0300
Cc: www-html@w3.org
To: www-html-editor@w3.org
Message-Id: <89952905-B390-11D6-B677-003065B8CF0E@iki.fi>

The Namespace

The Draft specifies a new namespace. I think it would be nice to have 
some rationale for this design decision in the spec. Defining a new 
namespace certainly makes it easier for implementors of XHTML 2.0 
user agents to resist requests to support legacy elements. However, 
for user agents that support both XHTML 1.x and 2.0, it would be 
easier to keep the same namespace and rename the elements whose 
semantics have changed.


External Entities and User Agent Performance

The Draft makes the inclusion of a doctype declaration in the 
document a "must" requirement and requires the PUBLIC id (if present) 
to reference a particular DTD. (However, the spec doesn't exactly 
require a lone SYSTEM id to reference an equivalent DTD resource, but 
I assume it is the intent.) I think this may cause performance 
problems to user agent that are able to parse DTDs if the namespace 
attributes are going to be handled the same way they are handled in 
XHTML 1.1.

In XHTML 1.1 each element has the xmlns attribute and the attribute 
has a #FIXED value. If each and every element in the document 
instance doesn't have the attribute explicitly, it will be provided 
via attribute defaulting if the external subset is processed. 
However, if the external subset is not processed, the attribute 
values won't be there. From the point of view of the Namespaces in 
XML spec, the result is the same, but from the point of view of XML 
1.0 validity constraints it makes a difference: if the documents was 
declared standalone difference in attribute defaulting when 
processing the external subset and when not would be a violation of a 
validity constraint.

As a result, if one wanted to construct a valid standalone document 
when the DTD has xmlns attribute defaults for each element, one would 
have to repeat the xmlns attribute on each element, which is 
obviously impractical. However, being able to declare documents 
standalone would be useful in terms of user agent performance. 
Fetching a DTD with all the modules or even loading and parsing them 
from a local catalog causes a noticeable performance hit for user 
agents. (This can be seen when comparing DocZilla and Mozilla for 
example. DocZilla actually parses the DTD but Mozilla doesn't.) 
Therefore, it would make sense indicate to user agents that they may 
safely leave the external subset unparsed and that the external 
subset is only referenced for the purpose of validation.

If the XHTML 2.0 spec requires the doctype declaration and uses 
attribute defaults similar to those in XHTML  1.1, it will be 
impractical to make valid standalone documents.


Content-Type

The Draft doesn't say which media type should be used for labeling 
the document instances when the transport used media type labeling. I 
think making the media type labeling clear early on is crucial for 
interoperability of implementations. It would also be good to include 
an explicit "must not" against sending XHTML 2.0 as text/html.


Entity references

The definition of entity reference implies that the DTD will declare 
character entities in addition to the predefined ones. I think doing 
so is unnecessary since XML allow the use of any Unicode character 
directly as UTF-8. Character entities move the responsibility of 
being able to deal with character aliases to the rendering end even 
though it is more of an input issue which should be dealt with at the 
data entry time. If someone really has to use an ASCII editor instead 
of a proper Unicode editor, the NCRs are there. On the other hand, 
allowing character entities makes it necessary to parse the external 
subset and that would complicate lightweight user agents with 
non-validating parsers unnecessarily.


Classes

The class example is ill-formed. <p class="note">...</p> would look 
better than using <span>.


accesskey

The Draft says 'Apple systems, one generally has to press the "cmd" 
key in addition to the access key.' The command key is the 
accelerator key for the keyboard shortcuts of the browser's own 
functions on Mac, so another modifier key (eg. ctrl) would have to be 
used in order to avoid conflicts.


Deprecated Elements (like br)

Since XHTML 2.0 defines a new namespace, there are no pre-existing 
elements in the namespace. The deprecated elements are effectively 
*created as deprecated*. I think creating elements as deprecated 
doesn't make sense, since that would mean creating a burdensome 
legacy where none would otherwise exist. On the other hand, if it is 
considered necessary to keep the deprecated elements, I think it 
would make more sense to keep them in the XHTML 1.x namespace.


Headings

I like the <section> and <h> arrangement a lot. However, I think 
including <h1> through <h6> unnecessarily complicates things. I'd 
like to suggest including only one way of marking up headings (the 
<h> and <section> way) instead of including two incompatible ways.


Quote

I think dropping the <q> element in favor of <quote> is a very good 
thing. In practice, generating context-sensitive quotation marks in 
the user agent is really hard to get right.

The Draft says: "Visual user agents are not required to add 
delimiting quotation marks - -". I think it would be better to make 
the statement stronger in order to avoid cases where both the author 
and the user agent add quotation marks. I suggest substituting "are 
not required to" with "must not".


Anchors

Like many others, I was surprised to find that the Draft uses a 
linking method of its own instead of simple XLink. Isn't simple 
XLinks supposed to be used in new specs exactly in the cases like <a 
href="...">?


The Edit Module

"This element is unusual for XHTML in that they may serve as either 
block-level or inline elements (but not both)." I think the dual 
nature of ins and del is quite undesirable. The rule "The del element 
must not contain block-level content when it is behaving as an inline 
element." can't be enforced in validation. Also, if an element has a 
dual inline/block nature, it is more difficult to handle the 
presentation of the element in a user agent style sheet. I think it 
would be more straight forward to have separate elements for block 
and inline deletions and insertions (just like div and span are 
separate).


Referencing Style Sheets

The definition of <link/> includes the old HTML 4 style sheet 
linking. Since there is a general processing instruction for 
associating style sheets with XML documents, I think requiring user 
agents to support <link/> style sheets is unnecessary.


The Metainformation Module

The Draft says that http-equiv exist for the purpose of HTTP servers 
gathering information for HTTP headers. I could be mistaken, but when 
it comes to HTML and XHTML 1.x there don't seem to be actual servers 
implementing this feature. I think the unimplementedness suggests 
that the feature has failed in practice and could be removed from 
XHTML 2.0.

There are browsers that pay attention to HTML http-equiv, and the 
http-equiv in HTML is routinely used for three purposes other than 
information gathering on the server side:

1) Trying to specify the charset parameter of the Content-Type 
header. I think this should not be supported in XHTML 2.0, since 
supporting this feature requires scanning the incoming data buffer 
before parsing since the information about the character set that is 
supposedly in an attribute can't be found in attribute parsing, 
because the information is needed before parsing. Also, since the 
character encoding issue is deal with at the XML level, it would be 
harmful to add another and less elegant way of specifying the 
character encoding.

2) Trying to make a "redirect" ("meta refresh") without knowing about 
real HTTP redirects.

3) Trying to manipulate cache behavior. Compared to proper HTTP 
headers this approach is harmful, because HTTP caches won't see the 
pseudo-HTTP header that the author thinks have some meaning to 
caching systems.

Then recently some authors have thought that including the tag <meta 
http-equiv="Content-Type" content="application/xhtml+xml" /> would do 
something good when the real HTTP header says text/html.

I think it would be appropriate to drop the http-equiv attribute. Or 
if is kept in XHTML 2.0, I think it would be good to include some 
notice that authors mustn't expect http-equiv attibutes to have any 
useful effect unless their server actually gathers information from 
the http-equiv attributes.


The Scripting Module

There's an example with document.write(). The way document.write() 
works in text/html user agents and is used is very tag soupish. 
Parsing the markup is suspended and a script prints strings to the 
tag soup parser input stream.

The XML parser is usually developed separately from the application 
using it. This is a good thing, since it allows the development of 
robust and reusable XML parser. It also makes implementing something 
like document.write() harder, which I think is a good thing, too. 
Implementing document.write() would likely require tampering with the 
separately developed XML parser or would require the use of a 
separate pseudo-XML parser in addition to the real parser so that the 
application could combine the element trees coming from the 
pseudo-XML parser with the main tree coming from the real parser.

I'd like to suggest disallowing the use of document.write() with 
XHTML 2.0 and with XML-based languages in general. This would 
simplify the implementation of user agents in other ways as well: 
When there is no document.write() there is no need to allow script 
elements to occur as descendants of the body element and there is no 
need to begin the execution of scripts before the entire document has 
been parsed and the corresponding DOM tree fully created.

Also, the script element has an attribute called charset for 
indicating the character encoding of an external script. I can't find 
a good description of the attribute. It seems to me that an author 
could use such an attribute for two purposes: to try to override the 
charset parameter of the Content-Type header of the script or to let 
user agent make a decision about not loading scripts whose characters 
are encoded in an unsupported way. I think that in the former case 
the author should be encouraged to get the real HTTP charset of the 
script itself right. As for the latter case, I'm inclined to think 
that the usefulness of the attribute would be minimal, because 
programming languages tend the be representable in common encodings.


Ruby

The Draft references Ruby. The Ruby spec doesn't say clearly what the 
proper namespace URL for the Ruby elements is, but in XHTML 1.1 the 
Ruby elements seem to be in the http://www.w3.org/1999/xhtml 
namespace. Since the module is unchanged in XHTML 2.0, it would be 
reasonable to assume that the elements are still in the 
http://www.w3.org/1999/xhtml namespace. Are they or does the 
http://www.w3.org/2002/06/xhtml2 get elements with the same local 
names and identical semantics?


Things That Weren't There

I've observed that the elements available in HTML and XHTML 1.x are 
structures that tend to appear in technical articles but that (X)HTML 
lack named elements for many structures that appear on Web pages.

Many Web pages include some kind of footer after the main content. 
The footer tends to contain the address of the author, a copyright 
notice, the date of update, a couple of works about the author and 
things like that. In HTML, one could write:
<hr>
<div class="footer">
<p>There author will be on vacation next week, so there won't be a 
new column next week. Last updated: 2002-08-17.</p>
</div>

The use of footers is so common that I think footers would deserve an 
element of their own:
<!-- no hr needed -->
<footer>
<p>There author will be on vacation next week, so there won't be a 
new column next week. Last updated: 2002-08-17.</p>
</footer>

Another thing that I've noticed is that (X)HTML doesn't provide any 
semantic markup for indicating which part of the page are main 
content and which parts are navigation. Usually news sites and the 
like have a lot of navigation alongside the main content. When using 
handheld user agents or tty user agents, it may be difficult to 
scroll around. I think it would be could be useful for these browsing 
situations as well as for styling to provide semantic markup for 
designating something as being part of the main content and something 
else as being part of navigation.

This would allow easy switching between the main content and the 
navigation parts in handheld ad tty clients. Also, providing a common 
way of marking up these thing would make it easier to write user 
style sheets that applied user preferences to the main content while 
leaving the navigation the way the author had suggested.

--
Henri Sivonen
hsivonen@iki.fi
http://www.hut.fi/u/hsivonen/
Received on Monday, 19 August 2002 12:28:00 GMT

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