Back in the SGML days, we had things called "viewers" to render markup
documents on the screen or on paper. Of course now, we all have Web
browsers instead.
An SGML viewer was (I say "was" rather than "is" for reasons that will
soon be apparent) a bit like a Web Browser in that it interpreted
markup, applied styling, and displayed the results on the screen. Some
of the fancier tools such as SoftQuad Panorama and CiTEC Multidoc Pro
allowed you to do all sorts of nice things with table layout,
collapsible tables of contents, footnotes, hypertext...the full wazoo.
Of course that was then and this is now. "Now" is a time and a place
when SGML viewers have been replaced with XML viewing capabilities in
Web Browsers. Neither Panorama nor Multidoc Pro are available any more.
I built a system three years ago that used Multidoc Pro and recently
had to revisit it. I needed to get Multidoc Pro but could not find it
anywhere on the Web. I ended up pulling it off a CD-ROM stuck in the
back of the Japanese version of my (now aged) book, "XML by Example"!
This viewer technology would seem to have been airbrushed out of
history; however, the brains powering Panorama and Multidoc Pro is a
component called Synex ViewPort, which is still available as a
developer toolkit (http://www.synex.se).
Theoretically, we no longer need viewers because browsers understand
XML and, in particular, Cascading Style Sheets and XSLT, either of
which can be used to create XML "viewers". As with so many things in
XML, it ain't quite that simple. Firstly, XSLT is really only a runner
for simple server-side styling; browser support is patchy at best; and,
as an XML standards, it has suffered from extensions that make keeping
applications browser independent difficult. Secondly, there is CSS2.
Although many browsers claim to support it, there are major CSS2
support and interoperability problems.
Over the last couple of weeks, I have downloaded more browsers than I
can remember in search of a decent CSS2 implementation. The nearest I
have come is Opera 5. After a lot of trial and error, I have an XML
browsing app working along similar lines to the Multidoc Pro apps I
used to build before XML. It has been an educational experience.
The most annoying thing about Opera is that it silently ignores errors
in your CSS2 stylesheet. There does not seem to be any way to force it
to report errors, a real pain in the neck during application
development. For example, I spent ages trying to figure out the one
particular style that was not working in the browser only to find that
I overstepped the maximum number of styles in my stylesheet and Opera
silently ignored a bunch of them!
The other very annoying thing is not a fault with Opera 5, but a fault
with XML. When XML was on the drawing board, the idea was to
standardize the markup language and then move quickly on to
standardizing a linking mechanism (XLink) and a styling mechanism
(XSL). Unfortunately, XLink has yet to get beyond working draft at the
W3C. Consequently, getting hypertext links to work in Opera requires
the use of Opera specific extensions to CSS2.
On the bright side, you can do a lot with CSS2 and, as support for it
amongst tools grows, it will become easier and easier to browse XML. We
will soon be in a position to do what SGML viewers could do 4 years ago.
Plus Ca Change, Plus C'est La Meme Chose.
