Re: SVG and MathML in text/html

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Re: SVG and MathML in text/html

Doug Schepers-3

Hi, Maciej-

I'm taking this discussion to www-archive, since it affects a lot of
groups and a lot of interests.  I'm BCCing related groups (HTML, SVG,
MathML, CDF, XHTML2), but discussions should take place on www-archive.
  (This is part of a couple of long threads that started on blogs and
IRC, and were continued on public-html; I'd suggest that those
interested in this review those threads. [1][2])

To broaden the dialog to other audiences, so I'm going to try to
summarize the issue along the way.  Please correct me if I fall astray,
or expand on points.  I'm going to talk about SVG, but many of the same
issues may apply to MathML.

Maciej Stachowiak wrote (on 3/16/08 1:12 AM):
>
> HTML has the feature of two serializations: a classic serialization that
> is error-tolerant, and an XML-based serialization that has draconian
> error handling. These have different costs and benefits, ultimately it
> is a benefit to HTML authors that they have a choice. I think SVG
> deserves to have this feature as well, there's no reason it should fall
> short of HTML in this regard. Supporting SVG inline in text/html seems
> like a good opportunity to add this feature to SVG.

There are 2 scenarios here: SVG inline (Compound Documents by
Inclusion), and external SVG (Compound Documents by Reference).  Your
proposal is that, in order to facilitate inline inclusion of SVG into
text/html, changes should be made to other uses of SVG, including those
intended for SVG-only UAs.  I'll ask you to detail what changes would be
required, but the topic of hottest debate so far is the ability to use
unquoted attribute values in SVG, when there are no spaces or other
ambiguous characters.  This would look something like this:

   <circle id=circle_1 class="category1 medium" cx=75 cy=25 r=20
fill=orange stroke=red stroke-dasharray="3 5" />

You characterize this as non-draconian error handling; for the purpose
of authoring conformance, would this fragment indeed be in error (and
therefor in need of error recovery behavior), or would this be legal
syntax?


A counter-proposal is that SVG retain its existing serialization
(meaning, among other things, that it would require quoted attributes),
for inline (in both XHTML and text/html) and for external SVG documents
(standalone or externally referenced).  I'm going to argue for this
proposal, as a Devil's Advocate, but I'm actually open to any proposals
that can be demonstrated as workable.


Given past author practice of copy-paste authoring, SVG that is written
inline in HTML is unlikely to stay there exclusively;  it stands a very
good chance of being extracted and reused, be it referenced as an
external image, or as standalone content in a mobile SVG-only viewer, or
pasted into a graphical SVG authoring tool.  Content that breaks from
one UA to another is too brittle, and would fracture the SVG market.
So, I agree with you here, that we shouldn't burden users, authors, and
vendors with multiple non-interoperable serializations.


As you mention, there are indeed known costs to any change in the
written form of SVG (the serialization), notably incompatibility with
all existing SVG viewers, authoring tools, automatic generators, and XML
processors up and down the entire toolchain in general.  There is an
enormous infrastructure investment involved in an enterprise like this,
which affects not just SVG and MathML, but XML in general, and which
would require substantial costs in money, developer hours, and time to
market.

On mobile devices in specific, this would require implementors to write
a new parser (which has yet to be shown as practical on the Web in
general, or with SVG in specific); this may not be a burden for Apple
(or certain other desktop browser vendors), which has already invested
resources in an HTML UA, but it would certainly be for multiple other
mobile vendors (those not on the iPhone), who have a very large (in the
upper hundreds of millions) existing SVG UA deployment, and this could
have a substantial cost for them.

For desktop browsers, your proposed content would not work in the most
widely deployed SVG UA plugin for Internet Explorer (the Adobe viewer,
which though no longer maintained, is still the best and fastest overall
SVG UA for browsers); again, I can see this being a benefit for Apple,
but not necessarily for SVG.  If Microsoft were to pledge and deliver
resources for adding high-quality native SVG support (at least as good
as Firefox, Opera, and WebKit), this would certainly alleviate my
concerns about desktop browsers.

With the XML serialization of SVG (the way it is now), authors are
already able to do inline SVG in the XML serialization of HTML (XHTML);
this works even in Internet Explorer using the Adobe viewer, by use of
"XML data islands"... and in fact works equally well in text/html and
XHTML (though it has to be served as text/html, a known problem in IE in
general).


Since it's not clear what changes would have to be made to SVG, your
proposal may or may not affect existing SVG content that is intended for
insertion into text/html; that is, presumably content can be made that
works in all SVG UAs, following the existing rules for SVG.  So, this is
a neutral point... neither proposal would break existing content from
existing SVG authoring tools and generators.  Still, it would be best to
get feedback from known tool vendors that produce SVG, such as Inkscape,
Illustrator, CorelDraw, GraphViz, Visio, and others (I'm probably
forgetting someone major, sorry).


As far as I know, there has been no implementation of the HTML5 parser
by a major browser vendor (though there have been test implementations
by others).  The algorithm is relatively untested on the vast body of
existing HTML content on the Web, and completely untested for SVG
content.  I'm concerned that this is taking an unnecessary risk with
SVG's future, in spite of its already growing appeal and popularity
across a variety of platforms, since it is competing with similar
proprietary technologies.  I argue that this is a bad time to be taking
risks for what I see as a largely theoretical benefit to authors.

As I've stated before, in 8 years of active participation in the SVG
community, I've heard no serious request for unquoted attributes; by
contrast, I have heard many people extolling the benefits of the cleaner
XML model in contrast to the more complex HTML legacy.  As it happens,
SVG makes liberal use of spaces in attributes, much more so than HTML,
so the analogies between the two are not quite accurate; the majority of
SVG content uses not the basic shapes like circles, lines, rectangles
and ellipses, but the more complex shapes represented by paths,
polygons, and polylines, and those elements use attribute values most
clearly represented by space-separated coordinate pairs.  It's not clear
that there would be a substantive gain by authors in real-world content
by excluding attribute value quotes.  Since SVG doesn't have the legacy
content of HTML content's unquoted attributes, there is little need to
extend the same rules to SVG as are necessary for HTML.

You state that "SVG deserves to have this feature as well" and that
"there's no reason it should fall short of HTML in this regard" as if
unquoted attributes are innately a clear benefit, rather than a burden
that implementors of HTML vendors and authors have to put up with,
because vendors have to deal with legacy content, and authors have to
maintain content they didn't create; SVG authors have never had to deal
with this before, and it's really not clear they want to.  You represent
this as an authoring benefit, but I have not seen the evidence that this
is something authors are requesting.  I may be wrong, but I'd like to be
proven wrong, rather than take it on faith.

In summary, as I see it, my proposal works today in a larger number and
wider variety of user agents, and imposes less burden on vendors, and
arguably on authors.


Could you summarize all the changes that you would require for SVG UAs
and authors to implement your proposal, and give the benefits and
rationales for each?


[1] http://lists.w3.org/Archives/Public/public-html/2008Mar/0039.html
[2] http://lists.w3.org/Archives/Public/public-html/2008Mar/0124.html

Regards-
-Doug Schepers
W3C Team Contact, SVG, CDF, and WebAPI