The Semantic Web

  1. What is the Semantic Web?
  2. Where did it come from?
  3. PICS, or Where has it been?
  4. RDF, DAML/OIL or Where is it up to?
  5. What does it do? Example - Accessibility

Charles McCN -

The Semantic Web??

Using metadata to add (and extract) meaning

Part of the original design ideas of the Web

Now a W3C Activity in its own right

Let the computer do the work!

Some Key Technologies:

RDF, RDF Schema, EARL, Dublin Core, OIL, DAML
XML architecture
XML, Namespaces, XLink, Xpath, XML Schema
XML languages
As well as
Annotea, CC/PP, PICS, implementation

Visions from 1994:

information that makes sense to people...

pages with links to each other

can be given semantics...

to let the computer add meaning for us.

When the links have meaning, the information can provide services to people, and more.


What is it?

In Principio (of the Web)...

From HTML - IETF draft 1.2 of 1993

element link - this document links to that one

Attribute rel="X"
The link to that document is of type X
E.g. <a href="gloss" rel="glossary">xxx</a.>
xxx is linked to something that is a glossary.

rel, rev are literal text values, for documents or hypertext

old technology: PICS

level is 0, health is 9, colour is 27

often but not always gives a "minimum or maximum"

Works easily in major browsers

Includes a trust mechanism...

PICS is useful...

Ratings bureaus allow third parties to add metadata

... but PICS is limited

name = value pairs

Hard to describe relationships

Schemes become very complex

RDF has (a bit) more power

RDF consists of "triples" of Information Subject:



is the


something else

In RDF we can...

Relate anything that can have a URI

Have literal values (for the 3rd term of a triple)

Use anonymous nodes or first-class objects

Take some information...

a set of circles and arrows

...and add to it

more circles and arrows superim


XML, RDF, Ontology vocabulary,

RDF+Schema layer

Very wide interoperability of RDF data

"Ontology" layer

Wide interoperability & interconversion

What is Accessibility?

Making the Web useful to all people

"Transform Gracefully"

equivalent output from different devices

Different input or output

CC/PP - an RDF framework for multiple devices

Equivalent alternatives, and metadata, to specify related pieces

Assistive technology to render to the users

Conversion of languages

PICS helps access...

... but mostly for rating afterwards

More powerful: RDF

Take an SVG image that includes some RDF....

An SVG for a computer network

Re-render the RDF...

A beta project: text-only RDF-aware SVG browser

Cable A connects Computer A and socket 1

Cable B connects Computer B and socket 2

Cable N connects external Network and socket 5

the Hub connects socket 1 and socket 2 and socket 3 and socket 4 and socket 5

Or re-render the RDF...

The metadata represented graphically

Combining technologies

Some XSLT or CSS stylesheet can be used to transform Semantic content

Text is difficult too...

Surprise A cartoon of an open-mouthed face is a symbol for the idea surprise

Available as a video is a symbol for the idea Available as a video

The theory of relativity is (available as a video)  : the video.

EARL - accessible authoring support

Evaluation And Report Language

Assesment of a particular requirement (Accessibility or not):


EARL pictured

on 2001/06/23 at 23:40Z Charles says:

My document earl:conforms

EARL - use case (authoring)

The content team puts together a news item

Three assessment tools are used, because tool B does some bits well but not others

EARL - use case (publishing)

The sub-editors' tools check for existing reports

Each repair is noted and need not be checked again until something changes

Thank You! Questions?

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