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Re: Wish for MathML 3: version attribute

From: Bert Bos <bert@w3.org>
Date: Sat, 15 Sep 2007 19:34:44 +0200
Message-ID: <46EC17B4.2030608@w3.org>
To: www-math@w3.org
CC: max@berger.name

Robert Miner wrote:
> I also support this.  I don't think it will be controversial.

Well...

If we want MathML2 and 3 to co-exist (rather than recommend everybody to
upgrade to 3) and the two versions require different implementations,
then the difference should be expressed in the MIME type, by means of a
new identifier or a parameter. Otherwise the client cannot tell the
server what it supports and the server cannot give the client what it needs.

Even if the server only has one version of the document, it would be
wasteful to send it to a client that in the end cannot handle it.

Also, we have to see what existing version-2 clients do with a version
attribute. If they don't stop on seeing the attribute but continue
parsing anyway, the attribute has no use.

Probably, there are also quite a few documents that would be valid in
both MathML2 and MathML3, apart from that version attribute. The version
attribute would require you to make one file for old clients and another
for new ones. Keeping the version information outside the document
avoids that.

In general, version information (in whatever form) embedded in the
document itself is not useful on the Web. Either the format has built-in
forward and backward compatibility (old implementations can do something
useful with new documents and vice versa), or it is in fact two
different formats and their identifiers should be available as MIME
types outside the document.

HTML has long hesitated over this. The official specification has until
now had version information in the document, but implementations ignored
it. HTML5 therefore looks set to abandon the version info.

CSS has never had version information and has always tried to be
forwards and backwards compatible. Sometimes people ask for version
information, but when questioned, it always turns out that what they
really mean is a way to target a particular browser. Version info
wouldn't help, because there is no specification that corresponds to a
particular browser, if only because there is no specification that
describes the bugs in browsers.

(There is one counter-argument against new MIME types and that is that
authors sometimes don't have control over the MIME types of their Web
server, because their ISP is of too low quality. Some browsers therefore
don't rely on the lesser known MIME types, but instead use content
sniffing for commonly mislabeled types, i.e., they download the document
and try to guess its type from the content.)

I haven't made a list of the changes between version 2 and 3, but I'm
hoping that we can avoid removing features (except for any that never
worked in the first place) and can thus keep all existing MathML
documents valid in version 3. The version 2 spec can then be declared
obsolete and all software has to be upgraded, but all content can remain
unchanged. There is more content than software and software has to be
updated regularly anyway. There is no need for a new MIME type or a
version parameter then.



Bert
-- 
   Bert Bos                                ( W 3 C ) http://www.w3.org/
   http://www.w3.org/people/bos                               W3C/ERCIM
   bert@w3.org                             2004 Rt des Lucioles / BP 93
   +33 (0)4 92 38 76 92            06902 Sophia Antipolis Cedex, France
Received on Saturday, 15 September 2007 17:35:04 UTC

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