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2 Introduction to HTML 4.0

2.1 What is the World Wide Web?

TheWorldWide Web (Web) is a network of information resources. TheWeb relies on three mechanisms to make these resources readilyavailable to the widest possible audience:

1. A uniform naming scheme for locating resources on the Web (e.g., URIs).
2. Protocols, for access to named resources over the Web (e.g., HTTP).
3. Hypertext, for easy navigation among resources (e.g., HTML).

The ties between the three mechanisms are apparent throughoutthis specification.

2.1.1Introduction to URIs

Every resource available on the Web -- HTML document, image, videoclip, program, etc. -- has an address that may be encoded by aUniversal Resource Identifier, or "URI".

URIs typically consist of three pieces:

1. The naming scheme of the mechanism used to access the resource.
2. The name of the machine hosting the resource.
3. The name of the resource itself, given as a path.

Consider the URI that designates the current HTML specification:

   http://www.w3.org/TR/REC-html40/

This URI may be read as follows: There is a document available viathe HTTP protocol (see[RFC2068]),residing on the machine www.w3.org, accessible via the path"/TR/REC-html4/". Other schemes you may see in HTML documentsinclude "mailto" for email and "ftp" for FTP.

...this is text...   For all comments, please send email to    <A href="mailto:joe@someplace.com">Joe Cool</A>.

2.1.2Fragment identifiers

Some URIs refer to a location within a resource. This kind of URIends with "#" followed by an anchor identifier (called thefragmentidentifier). For instance, here is a URI pointing to an anchornamedsection_2:

http://somesite.com/html/top.html#section_2

2.1.3Relative URIs

ArelativeURI doesn't contain any naming scheme information. Itspath generally refers to a resource on the same machine as the currentdocument. Relative URIs may contain relative path components (e.g.,".." means one level up in the hierarchy defined by the path), and maycontainfragment identifiers.

Relative URIs areresolved to fullURIs using a base URI. As an example of relative URI resolution,assume we have the base URI"http://www.acme.com/support/intro.html". The relative URI in thefollowing markup for a hypertext link:

   <A href="suppliers.html">Suppliers</A>

would expand to the full URI "http://www.acme.com/support/suppliers.html",while the relative URI in the following markup for an image

   <IMG src="../icons/logo.gif" alt="logo">

would expand to the full URI "http://www.acme.com/icons/logo.gif".

In HTML,URIs are used to:

• Link to another document or resource,(see theA andLINK elements).
• Link to an external style sheet or script (see theLINK andSCRIPT elements).
• Include an image, object, or applets in a page, (see theIMG,OBJECT,APPLET andINPUT elements).
• Create an image map (see theMAPandAREA elements).
• Submit a form (seeFORM).
• Create a frame document (see theFRAME andIFRAME elements).
• Cite an external reference (see theQ,BLOCKQUOTE,INS andDEL elements).
• Refer to metadata conventions describing a document(see theHEAD element).

Please consult the section on theURI type for more information aboutURIs.

2.2 What is HTML?

To publish information for global distribution, one needs auniversally understood language, a kind of publishing mother tonguethat all computers may potentially understand. The publishing languageused by the World Wide Web is HTML (from HyperText Markup Language).

HTML gives authors the means to:

• Publish online documents with headings, text, tables, lists,photos, etc.
• Retrieve online information via hypertext links, at the click of abutton.
• Design forms for conducting transactions with remote services, foruse in searching for information, making reservations, orderingproducts, etc.
• Include spread-sheets, video clips, sound clips, and otherapplications directly in their documents.

2.2.1 A brief history of HTML

HTML was originally developed byTim Berners-Lee while atCERN, and popularized by the Mosaic browserdeveloped at NCSA. During the course of the 1990s it has blossomedwith the explosive growth of the Web. During this time, HTML has beenextended in a number of ways. The Web depends on Web page authors andvendors sharing the same conventions for HTML. This has motivatedjoint work on specifications for HTML.

HTML 2.0(November 1995, see[RFC1866])was developed under the aegis of theInternet EngineeringTask Force (IETF) to codify common practice in late 1994.HTML+(1993) andHTML3.0 (1995, see[HTML30])proposed much richer versions of HTML. Despite never receivingconsensus in standards discussions, these drafts led to the adoptionof a range of new features. The efforts of the World Wide WebConsortium's HTML Working Group to codify common practice in 1996resulted inHTML3.2 (January 1997, see[HTML32]). Changes from HTML 3.2 are summarizedinAppendix A

Most people agree that HTML documents should work well acrossdifferent browsers and platforms. Achieving interoperability lowerscosts to content providers since they must develop only one version ofa document. If the effort is not made, there is much greater riskthat the Web will devolve into a proprietary world of incompatibleformats, ultimately reducing the Web's commercial potential for allparticipants.

Each version of HTML has attempted to reflect greater consensusamong industry players so that the investment made by contentproviders will not be wasted and that their documents will not becomeunreadable in a short period of time.

HTML has been developed with the vision that all manner of devicesshould be able to use information on the Web: PCs with graphicsdisplays of varying resolution and color depths, cellulartelephones, hand held devices, devices for speech for output and input, computers with high or low bandwidth, and so on.

2.3 HTML 4.0

HTML 4.0 extends HTML with mechanisms for style sheets, scripting,frames, embedding objects, improved support for right to left and mixeddirection text, richer tables, and enhancements to forms, offeringimproved accessibility for people with disabilities.

2.3.1 Internationalization

This version of HTML has been designed with the help of experts inthe field of internationalization, so that documents may be written inevery language and be transported easily around the world. This hasbeen accomplished by incorporating[RFC2070], which deals withthe internationalization of HTML.

One important step has been the adoption of the ISO/IEC:10646standard (see[ISO10646]) as the documentcharacter set for HTML. This is the world's most inclusive standarddealing with issues of the representation of international characters,text direction, punctuation, and other world language issues.

HTML now offers greater support for diverse human languages withina document. This allows for more effective indexing of documents forsearch engines, higher-quality typography, better text-to-speechconversion, better hyphenation, etc.

2.3.2Accessibility

As the Web community grows and its members diversify in theirabilities and skills, it is crucial that the underlyingtechnologies be appropriate to their specific needs. HTMLhas been designed to make Web pages more accessible tothose with physical limitations. HTML 4.0 developments inspiredby concerns for accessibility include:

• Better distinction between document structure and presentation,thus encouraging the use of style sheets instead of HTML presentationelements and attributes.
• Better forms, including the addition of access keys, theability to group form controls semantically, the abilityto groupSELECT optionssemantically, and active labels.
• The ability to markup a text description of anincluded object (with theOBJECT element).
• A new client-side image map mechanism (theMAPelement) that allows authors to integrate image and text links.
• The requirement that alternate text accompany images includedwith theIMG element and image mapsincluded with theAREA element.
• Support for thetitle andlang attributes on all elements.
• Support for theABBR andACRONYM elements.
• A wider range of target media (tty, braille, etc.) foruse with style sheets.
• Better tables, including captions, column groups, and mechanisms to facilitate non-visual rendering.
• Long descriptions of tables, images, frames, etc.

Authors who design pages with accessibility issues in mind will not only receive the blessings of the accessibility community, but willbenefit in other ways as well: well-designed HTMLdocuments that distinguish structure and presentation willadapt more easily to new technologies.

2.3.3 Tables

The new table model in HTML is based on[RFC1942]. Authors nowhave greater control over structure and layout (e.g., column groups).The ability of designers to recommend column widthsallows user agents to display table data incrementally (as it arrives)rather than waiting for the entire table beforerendering.

2.3.4 Compound documents

HTML now offers a standard mechanism for embedding generic mediaobjects and applications in HTML documents. TheOBJECT element (together with its more specificancestor elementsIMG andAPPLET) provides a mechanism for includingimages, video, sound, mathematics, specialized applications, and otherobjects in a document. It also allows authors to specify a hierarchyof alternate renderings for user agents that don't support a specificrendering.

2.3.5 Style sheets

Style sheets simplify HTML markup and largely relieve HTML of theresponsibilities of presentation. They give both authors and userscontrol over the presentation of documents -- fontinformation, alignment, colors, etc.

Style information can be specified for individual elements orgroups of elements. Style information may be specified in an HTMLdocument or in external style sheets.

The mechanisms for associating a style sheet with a document isindependent of the style sheet language.

Before the advent of style sheets, authors had limited control overrendering. HTML 3.2 included a number of attributes and elementsoffering control over alignment, font size, and text color. Authorsalso exploited tables and images as a means for laying out pages. Therelatively long time it takes for users to upgrade their browsersmeans that these features will continue to be used for sometime. However, since style sheets offer more powerful presentationmechanisms, the World Wide Web Consortium will eventually phase outmany of HTML's presentation elements and attributes. Throughout thespecification elements and attributes at risk are marked as "deprecated". They are accompanied by examples of how to achieve the same effects withother elements or style sheets.

2.3.6 Scripting

Through scripts, authors may create dynamic Web pages (e.g., "smartforms" that react as users fill them out) and use HTML as a means tobuild networked applications.

The mechanisms provided to include scripts in an HTML document areindependent of the scripting language.

2.3.7 Printing

Sometimes, authors will want to make it easy for users to printmore than just the current document. When documents form part of alarger work, the relationships between them can be described using theHTMLLINK element or using W3C'sResource Description Language (RDF) (see[RDF]).

2.4 Authoring documents with HTML 4.0

We recommend that authors and implementors observe the followinggeneralprinciples when working with HTML 4.0.

2.4.1 Separate structure and presentation

HTML has its roots in SGML which has always been a language for thespecification of structural markup. As HTML matures, more and more ofits presentational elements and attributes are being replaced by othermechanisms, in particular style sheets. Experience has shown thatseparating the structure of a document from its presentational aspectsreduces the cost of serving a wide range of platforms, media, etc., andfacilitates document revisions.

2.4.2 Consider universal accessibility to the Web

To make the Web more accessible to everyone, notably those withdisabilities, authors should consider how their documents may berendered on a variety of platforms: speech-based browsers,braille-readers, etc. We do not recommend that authors limit theircreativity, only that they consider alternate renderings in theirdesign. HTML offers a number of mechanisms to this end (e.g., thealt attribute, theaccesskey attribute, etc.)

Furthermore, authors should keep in mind that their documents maybe reaching a far-off audience with different computerconfigurations. In order for documents to be interpreted correctly,authors should include in their documents information about thenatural language and direction of the text, how the document isencoded, and other issues related to internationalization.

2.4.3 Help user agents with incremental rendering

By carefully designing their tables and making use of new tablefeatures in HTML 4.0, authors can help user agents render documentsmore quickly. Authors can learn how to design tables for incrementalrendering (see theTABLEelement). Implementors should consult thenotes on tables in theappendix for information on incremental algorithms.