The explosive growth of the World Wide Web since its inception in 1993 has seen growing expectations from the user base, in terms of the capabilities of tools and the means of representing information. First generation web standards, such as early variants of HTML and those beloved mainstays of web imagery, JPEG and GIF87/89, are less than ideal for the delivery of the kind of content many users now seek.

Of particular importance are the limitations of existing standards in controlling the presentation of data and imagery, the frequently poor density of imagery data, but also the difficulties experienced in machine driven analysis of web content.

The latter has proven to be a major issue, as it limits what can be usefully done by automated systems such as search engines. We have all had the experience of typing in to a search engine what would appear to be unambiguous specifiers, only to be inundated with mostly irrelevant URLs. What is an annoyance to a human reader who has the intelligence and the ability to use heuristics to filter out irrelevant content, amounts to an extremely difficult if not intractable problem for automated system. Without the ability to use heuristics as a human being does, automated systems sink very quickly indeed.

Other problems with the existing HTML 1-3 based web standards are the cumbersome handling of mathematical expressions, a major hindrance to the wider use of hypertext for publishing scientific papers, and the frequently highly inconsistent presentation of markup across different browsers. We have all had the experience of trying to produce a web page layout only to find out that it doesn't quite look the same on browsers other than that used during the testing of the webpage.

Which of these are the most pressing concerns at this time ? Evidently, judging from activity in web standards development, all areas are considered to be serious issues.

In this month's feature we will briefly survey some of the current draft standards or recommendations being explored by the World Wide Web Consortium (W3C at http://www.w3.org/TR/), to provide the reader with some idea of current trends in web publishing standards.

Vector Graphics Standards

One of the most glaring inadequacies of the existing web standards base is the inability to support vector graphics particularly well. Vector graphics are images which are represented by mathematical descriptions of lines, shapes, curves, and areas, rather than fields of pixels set to individual colours.

Vector graphics have some highly attractive qualities for presentation:

The ability to cleanly translate the image into another vector representation with no loss of information and thus image quality. If an alternate vector representation standard can support the same graphics primitives, an exact mapping can be performed.

There are many vector graphics based file formats in existence, examples being Postscript/EPS, Computer Graphics Metafile (CGM), HPGL, Adobe Illustrator and a host of others.

Vector graphics representation is the basis of all engineering drawing packages, and it is by far the best way of representing charts, plots, graphs, line drawings and line illustrations.

For a web environment, this technique is close to ideal, since the quality of the image is limited primarily by the quality of the browser and the display or printer. The currently widespread use of GIF and JPEG for this purpose usually does a disservice to most such imagery.

At this stage two schemes are being explored as the basis of future web standards.

The first is the Scalable Vector Graphics (SVG) language, derived from the XML standard. The aim of SVG is to provide a genuine vector graphics language, capable of representing conventional graphical shapes, such as lines, areas, curves, text and embedded bitmap images, which can be grouped, styled, transformed and composited. Facilities exist for shading and colour transitions. The SVG model goes further than conventional vector representations, since it embeds features allowing access to global attributes, as well as providing facilities for scripting. Animation can be incorporated, extending the established XML model.

SVG is now at the Candidate Recommendation phase in the W3C standards scheme, while effort continues on the related SMIL animation scheme.

Another proposal proceeding through the same standardisation system is the WebCGM standard, a web optimised subset of the widely used ISO Computer Graphics Metafile standard (ISO/IEC 8632:1992) vector graphics standard. CGM has been a mainstay of the CAD/CAM industry, and also is the only industry standard vector format accepted by most Microsoft tools (... hint for xfig users who need to submit work in MS Word or PPT).

The attractiveness of WebCGM lies in its huge established industry base, which makes the development of robust output filters a very economical proposition, since it amounts to little more than tweaking existing and often very mature code. The WebCGM proposal is largely derived from the CGM Open consortium (see http://www.cgmopen.org/) CGM model, and the ATA (Air Transport Association) CGM profile, the latter was simplified to conform to existing W3C standardisation requirements.

While SVG is a newer and arguably a more flexible scheme than WebCGM, the latter is almost off-the-shelf. Incorporation of either or both into future browsers will provide an unprecedented increase in the quality of web graphics, and hopefully will also see the eventual disappearance of those tedious, oversized GIFs and JPEGs so popular with graphics rich websites.

Bitmap Graphics Standards

Another important graphical standard to recently emerge is the Portable Network Graphics (PNG) standard, designed to replace the GIF87a and GIF89a standards, without the legal encumbrance of the compression standard in GIF. PNG is however a much more sophisticated standard than GIF, and in some respects is considered a replacement for the Adobe TIFF format.

PNG retains many of the nice properties of GIF, it supports progressive display during a download, transparency, lossless compression and supports 256 colour indexed images. However, it also provides new features, such as 48 bit per pixel true colour, 16 bit per pixel grayscale, a per pixel alpha channel for transparency information, embedded gamma correction to accommodate arbitrary displays, reliable error detection using a 16-bit CRC code and is faster to render initially than a GIF.

In many respects, PNG fills the gap between the legacy technology GIF, evolved in the era of 8-bit graphics adaptors, and JPEG, which as a lossy compression scheme frequently damages fine detail in bitmap images. Web page designers who are fussy about bitmap image quality, this writer inclusive, have long suffered the indignity of putting up with either miserable colour palletes in GIFs or loss of sharpness in JPEGs. PNG provides a TIFF-like large colour pallete, but does so with lossless compression to always retain high image quality.

Markup Languages - HTML 4.01

The mainstay of the current web is the trusty Hyper Text Markup Language, or HTML. A number of HTML versions remain in use, with sites being written around versions 1, 2 and 3. The latest incarnation of basic HTML is HTML 4.01, yet another incrementally improved variant of the basic product.

Like earlier versions of HTML, HTML4 is build around the well loved mechanisms of the HTTP protocol, hypertext, and the Universal Resource Identifier (URL for traditionalists). While earlier standardisation efforts were aimed primarily at forcing interoperability, HTML adds numerous pieces of extra functionality:

The extensions to the basic HTML we love and know so well are quite extensive in a number of areas.

The changes to the table scheme is built around the IETF RFC 1942 model, and is designed to add column groups and column widths, the latter to allow display as the table data is received by a browser.

The IMG and APPLET elements in earlier HTML are now replaced by the generic OBJECT, which is intended for displaying images, video, sound, mathematical equations, and includes provisions for alternate renderings where the browser cannot handle the intended rendering.

A big enhancement to HTML4 is the adoption of a style sheet mechanism to control the layout of a document. Style sheets will be either embedded in the HTML document, or provided via an external style sheet document, and will cover elements or groups of elements in a HTML document. With a style sheet mechanism, HTML authors will be able to locally and globally control attributes such as font information, alignment and colors in a HTML document.

Scripting support is improved in HTML4, the intend being to allow for dynamic form web pages which adaptively change as the reader types responses into them.

In perspective, the biggest gain to be seen from HTML4 is the style sheet mechanism, since it provides a means of putting some consistency into website presentation. This has been, arguably, the greatest weakness of the HTML markup mechanism to date.

XHTML 1.0 - The Extensible HyperText Markup Language

The next step in the evolution of HTML is XHTML, which in the simplest of terms is a reformulation of the HTML 4 standard in XML. The long term aim of the W3C standards community is a to XML, and XHTML is an important transitional phase in this process, since it provides a bridge between what will be established HTML 4 applications and the future generation of XML based browsers and production tools.

XML is a more powerful SGML derivative than the relatively lightweight HTML, itself also derived from SGML. SGML is considered to be extremely complex, and powerful, and its complexity has largely been the reason why it has not become widely adopted in practical tools.

HTML, as a minimal subset of SGML, could not keep up with the expectations of web users, and the strategy of migrating to XML is intended mainly to bypass the plague of proprietary enhancements to HTML, which has been a feature of the web in recent years. By providing a standard which is powerful enough to make all proprietary HTML variants irrelevant, the W3C aims to discourage proprietary players from contaminating the standard with incompatible modifications.

One of the basic aims of the XML standard is to make to easy to define new types of markup, and XHTML is intended to allow straightforward inclusion of XML additions into XHTML documents. Another important aim si to provide mechanisms which allow web servers to optimise the presentation of web site content, depending upon the type of browser and display device being used to access it.

The XHTML proposal describes three categories of compliance for a document: Strict, where only mandatory features are used, 'Transitional' and Frameset, which correspond to their HTML 4 equivalents. The XML namespace mechanism will be supported.

The XHTML proposal also details important differences from HTML4: