In the field of programming there’s universal appreciation for abstraction. It’s been beaten into programmers since day zero through the painful experience of changing brittle software full of code duplication and poorly expressed metaphors. Programmers learnt to remove and refactor duplication and to architect beautiful abstractions that identify and reuse commonalities in code.
The programmers were right about abstraction, but most have had a blind-spot when it came to HTML.
In the past few years there’s been increasing interest in the semantics of HTML and debate about interpretations of the HTML 4.01 standard. Menus that were previously in <table>s were moved to <ul>s. Then <table>s were relegated to tabular data and layout was achieved with the <div>ision tag. FAQ lists were expressed as <dl>, <dt>, <dd> and even the archaic <address> tag has been seen a comeback with it’s semantics more clearly defined than ever before. There comes a time however when pawing over the W3C specification will not reveal more semantics and when this particular markup language will not be sufficient for the demands that programmers put upon it.
And the demands are mounting. Pages should be able to be repurposed into syndication feeds (RSS 0.92/2.0, ATOM) which means programmers need article metadata. Interactive forms with validation should be available in HTML 4, HTML 5, Flash/Flex, XUL and XAML (with native controls where possible). Generating PDFs, ODTs and OOXML/DOCs would be nice, and data-tables within pages should be exportable to CSV or ODS.
HTML 4.01 is currently stirring the same kind of doubt in programmers that duplication in source code does. The abstraction in HTML 4.01 is insufficient. It seems wrong to code an application to produce HTML 4.01 when there are commonalities with HTML 5, XUL, Flash/Flex, XAML, etc. A series of tabs could be native in XUL and XAML but emulated with a hack of <div> tags in HTML. A slider control 
or a date picker could be native in HTML 5 but emulated in HTML 4. A programmer putting slider controls on a page shouldn’t code to a particular output format, they shouldn’t have to care.
Most programmers could easily imagine how an HTML 5 date picker could be automatically converted into HTML 4 and JavaScript but the reverse is not so easily conceivable. All the inconsistent hacks that HTML 4 pages use to emulate date pickers makes a conversion from HTML 4 to HTML 5 impossible for all but contrived and simplistic examples. Converting HTML 4 hacks will be considerably more difficult precisely because HTML 4 lacks standard semantics for expressing these widgets. The 4.xx series of HTML has been with us for so long that perceivably it has been a constant, and applications have been hard-coded to output HTML 4. Any abstraction from HTML 4 was considered over-engineering, widget hacks became common place, and the blind-spot grew.
The proliferation of JavaScript libraries to emulate widgets shows an inherent weakness in HTML 4 but the problem isn’t just in form widgets. Textural content also has limitations in HTML 4, as anyone who knows DocBook or TEI will tell you.
After recognising this problem of abstraction programmers are now looking for higher-level languages that can be converted into lower-level ones such as HTML 4, RSS, PDF, ODT, etc. This conversion may occur server-side with an abstract page definition… perhaps a source in HTML 5, Flex, XUL, or XAML that is converted into output formats such as HTML 4 with JavaScript, HTML 5 with JavaScript, XUL, XAML, RSS, ODT, PDF, and so on.
What would a source format that straddled all these formats look like? Well, the output formats all can represent tables, paragraphs, headings, images, text-boxes, date pickers, hyperlinks and so on. Some features clearly overlap. Beyond that it’s more difficult, and more debatable (do XUL and XAML have features that map poorly to HTML and SVG? Is an abstraction still useful without these features?) so I’ll save that for another blog post.
Of course the devil’s in the details, and the feasibility of straddling a useful subset will only be proven in the semantics chosen. Still, going through the process will be an interesting exercise.
What would wxWidgets for the web look like? Stay tuned…
(Please note: Although I specifically mention HTML 4 as being a poor abstraction or internal data-model for modern applications the same goes for XHTML 1 as they have the same semantics, obviously)
