|Accessibility Links and References|
|Vision and Visual Disabilities – An Introduction|
|SAP's Public Accessibility Website|
|Accessibility on SAP's Service Marketplace|
A general term used to describe how easy it is for people to get to, use, and understand things.
Accessibility is most often used to describe facilities or amenities to assist people with disabilities, as in "wheelchair accessible." This can extend to Braille signage, wheelchair ramps, audio signals at pedestrian crossings, walkway contours, Website design, and so on. (From www.wikipedia.org, adapted)
See Tab Chain
a group of interrelated Web
development methods used on the client-side to create asynchronous Web applications.
With Ajax, Web applications can send data to, and retrieve
data from, a server asynchronously (in the background) without interfering
with the display and behavior of the existing page. Data is usually retrieved
Despite the name, the use of XML is not needed , and the requests do not need
to be asynchronous.
Ajax is not one technology, but a group of technologies. HTML and CSS can
be used in combination to mark up and style information. The DOM is
display, and to allow the user to interact with the information presented.
XMLHttpRequest object provide a method for
exchanging data asynchronously between browser and server to avoid full page
reloads. (From Wikipedia,
HTML tag that provides alternative text when non-textual elements, typically images, cannot be displayed. (From www.marketingterms.com/dictionary/)
See also: ALT Text
Refers to "alternative text" that is placed in the code for an image in an HTML page (in an ALT tag). If the image is not displayed, the ALT text can be presented instead. ALT text is especially useful to users of speaking browsers. The text should be a brief representation of the purpose of the image, not a description of the image. ALT text is frequently seen on tooltips when users move the mouse over images.
A 1990 federal law that forbids discrimination against persons who are disabled. It gives civil rights protection to individuals with disabilities similar to those provided to individuals on the basis of race, color, sex, national origin, age, and religion. It guarantees equal opportunity for individuals with disabilities in public accommodations, employment, transportation, State and local government services, and telecommunications.
ARIA is a metadata specification that provides a means of describing roles, states, and properties for custom widgets so that they are recognizable and usable by assistive technology users. WAI-ARIA also provides a mechanism to ensure that users of assistive technologies are aware of updates in the application.
There are currently three key documents for ARIA available from the W3C:
Any item, piece of equipment, or system, whether acquired commercially, modified, or customized, that is commonly used to increase, maintain, or improve functional capabilities of individuals with disabilities. (From Section 508)
This definition is very broad, potentially including anything from wheelchairs to hearing aids. In the context of Section 508, the term typically refers to tools used by disabled individuals to increase or improve the functionality of information technology.
Assistive technology can be either software- or hardware-based. Hardware solutions can span a broad range of devices, from low-tech items such as mouth sticks or paper stabilizers, to high-tech products such as refreshable Braille displays or eye gaze communication systems. Software solutions can include built-in features such as the accessibility options in Microsoft Windows or stand-alone programs such as speech recognizers and screen readers. The most appropriate type of assistive technology for a disabled individual depends on the nature of the disability.
Stands for "Barrierefreie Informationstechnik-Verordnung" = barrierfree Information technology regulation. It is an enhancement to the German "Behindertengleichstellungsgesetz." All regulations apply to Web presences and public accessible services on the internet offered by public authorities. BITV clauses are based on Web Content Accessibility Guidelines 1.0. ( -> see WCAG). BITV goes along with WCAG priorities 1 and 2. Central navigation- and access services even have to fulfill priority 3 requirements (§3 BITV).
Means that people cannot see or recognize anything. This includes cases where people may have a "global" brightness impression only. Blindness can be congenital, or appear later during the life span– then caused by accidents, diseases, or the aging process.
A tool designed to test whether or not a Website complies with accessibility guidelines for the visually impaired. Sites could gain a "Bobby approved" logo to display, which confirmed their compliance.
CAST launched Bobby as a free public service in 1995, but no longer supports it. Bobby was sold to Watchfire in 2004 which, in turn, was acquired by IBM in 2007. Although Bobby is no longer available as a free service or standalone product, it is one of the tests included within the IBM Rational Policy Tester Accessibility Edition software, an enterprise application for testing Websites.
A code which enables blind persons to read and write. It was invented by a blind Frenchman, Louis Braille, in 1829. Braille is comprised of a rectangular six-dot cell on its end, with up to 63 possible combinations using one or more of the six dots. Braille is embossed by hand (or with a machine) onto thick paper, and read with the fingers moving across on top of the dots. Combinations of Braille dots within a cell represent contractions of two or more print letters and Braille characters take up three times as much space as print.
These displays create a tactile translation of information on a computer screen. Some Braille displays have a reusable, refreshable surface, composed of rounded pins that rearrange to translate information as it is selected on screen.
See also: Refreshable Braille Display
A computer keyboard with braille-coded keys. For example, transparent Braille labels allow both blind and sighted users to access a keyboard.
The software on a computer that allows Websites to be rendered so they can be "read" by users, this may be a browser that renders things visually, in a manner confined to text only, or in any other manner that may be appropriate, such as voice output.
A standard for specifying the appearance of text and other elements. CSS was developed for use with HTML in Web pages but is also used in other situations, notably in applications built using XPFE. CSS is typically used to provide a single "library" of styles that are used over and over throughout a large number of related documents, as in a web site. A CSS file might specify that all numbered lists are to appear in italics. By changing that single specification the look of a large number of documents can be easily changed.
Cataracts are a cloudiness that is formed in the lens of the eye and may cause poor vision. Most cataracts result from aging and long-term exposure to ultraviolet light. Cataracts get worse over time – the clouded areas become larger and denser. The time taken for this to happen varies from a few months to many years. Cataracts are the leading cause of vision loss among adults of 55 and older.
Read more at Guide Dog Association: www.guidedogs.com.au/quality-of-life/common-vision-impairments/cataracts
People with cognitive disabilities (impairments) may have trouble reading, for example, auditory processing deficits and attention deficit disorders.
These deficiencies belong to the receptor-related impairments and refer to the inability to see colors faithfully or at all. The following two color deficiencies are caused by a malfunction of one or two of the three cone systems:
The retina in our eyes contains two basic types of light sensors (photoreceptors), cones and rods, which form independent visual systems that are dedicated to special tasks. Cones are primarily used for color and daylight vision.
For blind users and users with visual impairments, textual descriptions, also called text equivalents, can be provided that are read aloud by a screen reader. Text descriptions can be provided for user interface elements, such as controls, as well as for images and animation (required by Section 508 and the Web Content Accessibility Guidelines).
Adding Descriptions to Elements: In HTML 4.0, every element can have a tooltip that is displayed when the mouse rests over the corresponding element; it can be read to blind people while an element is focused by Screen Reader software. You can set the tooltip text with the title attribute of the element.
Text descriptions for controls should contain the following elements:
When using images, graphical representations of text, frames, image maps, applets, or animation, you can also use the alt attribute for this purpose. If necessary, as in the case of complex graphics, also maintain a long description in the longdesc attribute. This description refers to a URL that leads to further information about the element.
A design philosophy, which aims to improve the life of everyone through design. (From www.design-for-all.org, adapted)
Deuteranopia is a malfunctioning in the green cone system. People with deuteranopia are dichromats having only one malfunctioning cone system. They cannot distinguish green from certain combinations of red and blue. This is the most common type of color deficiency.
DHTML allows scripting languages to change variables in a Web page's definition language, which in turn affects the look and function of otherwise "static" HTML page content, after the page has been fully loaded and during the viewing process. Thus the dynamic characteristic of DHTML is the way it functions while a page is viewed, not in its ability to generate a unique page with each page load. (From Wikipedia, adapted)
People with only one malfunctioning cone system can have the following impairments:
Some people suffer from decreased mobility. This can range from stiffness of fingers due to arthritis to complete paralysis below the neck.
A condition that curtails to some degree a person's ability to carry on his normal pursuits. A disability may be partial or total, and temporary or permanent.
A name shared by two laws passed in Australia (1992) and the United Kingdom (1995). They both prohibit discrimination against people with disabilities.
The Document Object Model (DOM) is a cross-platform and language-independent convention for representing and interacting with objects in HTML, XHTML and XML documents. Aspects of the DOM (such as its "elements") may be addressed and manipulated within the syntax of the programming language in use. The public interface of a DOM is specified in its application programming interface (API). (From Wikipedia, adapted)
The selective impairment of reading skills despite normal intelligence, sensory acuity, and instruction (also called developmental dyslexia). Several perceptual studies have suggested that dyslexia subjects process visual information more slowly than normal subjects. Such visual abnormalities were reported to be found in more than 75% of the reading-disabled children tested. Therefore, it is important to rule out problems with sensory acuity (including visual acuity and visual processing) before labeling an individual as truly dyslexic.
The indication and setting of the actual focused element is essential both for blind and non-blind users using the keyboard. For people with visual disabilities, a method should be provided to enlarge/modify the focus indicator (typically: a dashed rectangle).
There should be a default focusable element on each page and focus should be set to this element when navigating to this page in case the entire screen changes, for partial screen changes, the focus should remain on the trigger element.
A feature of HTML that allows a Web author to divide a page into two or more separate windows. If the frame does not have a <title> element, or the <title> element is not meaningful this can cause accessibility issues. In addition, some (older) Web browsers do not support frames.
A neural-related impairment, is often caused when the fluid in the eye does not drain away fast enough. The built up pressure damages the optic nerve and prevents visual information from reaching the brain. Glaucoma typically affects peripheral vision first (tunnel vision), and can cause total blindness if not treated at an early stage.
A device attached to the user's head that allows a user to move the pointer on the screen, (usually moved by the mouse, a pointing device, the arrow keys, or with the movement of their head). It is useful for people who have limited mobility.
People with these impairments may be completely deaf, or may have partial loss of hearing.
HTML is written in the form of HTML elements, which are the basic building-blocks of Web pages. The elements consist of tags, enclosed in angle brackets (like <html>), within the Web page content. HTML tags most commonly come in pairs like <h1> and </h1>, although some tags, known as empty elements, are unpaired, for example <img>. The first tag in a pair is the start tag, the second tag is the end tag (they are also called opening tags and closing tags). In between these tags, Web designers can add text, tags, comments and other types of text-based content. (From Wikipedia, adapted)
A generic name for all of the technologies involved with communicating with computers.
A worldwide computer network using the TCP/IP protocols (and other protocols like HTTP on top of it). The World Wide Web, for example uses HTTP (Hypetext transmission protocol).
A computer network that uses Internet Protocol technology to securely share any part of an organization's information or network operating system within that organization.
The term is used in contrast to Internet, a network between organizations, and instead refers to a network within an organization. Sometimes, the term refers only to the organization's internal Website, but may be a more extensive part of the organization's information technology infrastructure. It may host multiple private Websites and constitute an important component and focal point of internal communication and collaboration. (From Wikipedia, adapted)
An object-oriented programming language developed by Sun Microsystems expressly designed for use in the distributed environment of the Internet. It was designed to have the "look and feel" and object-oriented style of the C++ language, but it is simpler to use than C++. Java can be used to create complete applications that may run on a single computer or be distributed among servers and clients in a network. It can also be used to build a small application module or applet for use as part of a Web page.
A popular screen reader software program (Windows only). It uses a synthetic voice to read a computer screen out loud.
The most important accessibility feature to be provided for Web pages. It allows users to access each "active" element in the portal with the keyboard. Keyboard accessibility is especially important for physically impaired and blind users who cannot use the mouse. All other users will appreciate this feature, too, especially advanced users who enter mass data, or users who work with a laptop.
Most Web application pages contain forms that consist of labels and input elements, such as input fields. The labels are simple text elements that describe the input elements. With HTML 3.2 the W3C introduced a special label element to be used in this case. This new element offers the advantage that an input field and its label are directly connected. The label's attribute for establishes the connection. Screen readers will then be able to read the correct label whenever an input element gets the focus.
A generic term that refers to a heterogeneous group of disorders manifested by significant difficulties in the acquisition and use of listening, speaking, reading, writing, reasoning or mathematical abilities. These disorders are intrinsic to the individual and presumed to be due to Central Nervous System Dysfunction. Even though a learning disability may occur concomitantly with other handicapping conditions (e.g. sensory impairment, mental retardation, social and emotional disturbance) or environmental influences (e.g. cultural differences, insufficient/inappropriate instruction, psychogenic factors) it is not the direct result of those condition or influences.
A visual impairment, not corrected by standard eyeglasses, contact lenses, medication, or surgery, that interferes with the ability to perform everyday activities. (Definition by National Eye Institute)
Belongs to the receptor-related impairments and is damage to or breakdown of the macula and thus a loss of the central vision. Color vision may also be diminished, although peripheral vision and night vision usually remain unaffected.
Macular degeneration may be the result of aging processes in the eye (age-related maculopathy, ARM, also called senile macular degeneration); there are also some other forms of macular degeneration, which are inherited and not associated with aging.
People with two or three malfunctioning cone systems can be assigned to the following two types:
A Microsoft technology (Microsoft® Active Accessibility®, MSAA) available as an add-on since Windows 95 and built into subsequent Windows releases that provides a standard, consistent mechanism for exchanging information between applications and assistive technologies. For example, MSAA allows applications to expose screen readers to the type, name, location, and current state of all objects and notifies screen readers of any Windows event that leads to a user interface change.
There a two types of neural-related visual impairments:
Belongs to the receptor-related impairments and is a lack of the ability to see at night. It sets in if light intensity goes below a certain level, for example, at the twilight level.
Night-blindness is caused by malfunctions of the rod system. The cone system alone is not sensitive enough for night vision (cones comprise only about 1/25 of the number of rods). The rods may be affected in many ways, one common example is a breakdown of the pigments (retinitis pigmentosa).
Developed by Adobe Systems Inc., as a way to publish documents electronically, with good formatting for printing, and document security (documents are generally read only). Originally, it was in an image format, and this presented major accessibility issues. Recently however, Adobe has made large strides in making the PDF format accessible to people with disabilities.
A module (either hardware or software) that adds a special feature to a larger system or program. For example, a program to allow a browser to play movies or to display Flash content.
Photophobia is a symptom of abnormal intolerance to visual perception of light (extreme light sensitivity). As a medical symptom it is not a morbid fear or phobia, but an experience of discomfort or pain to the eyes due to light exposure or by presence of actual physical photosensitivity of the eyes, although the term is sometimes additionally applied to abnormal or irrational fear of light such as heliophobia. (From Wikipedia, adapted)
Protanopia is a malfunctioning in the red cone system. People with protanopia are dichromats having only one malfunctioning cone system. Typically, they can distinguish only two (yellow, blue) or three colors (yellow, blue, purple) – yellow comprises red, orange, yellow, and green, blue coincides with blue and purple.
The following types of receptor-related impairments have been described:
An electro-mechanical device for displaying Braille characters, usually by means of raising the dots through holes in a flat surface. Refreshable braille displays sit under a regular computer keyboard, and smaller displays (usually 18-40 cells) are available on some models of note takers.
See Section 508
Belongs to the receptor-related impairments and is a congenital inherited disease, which causes the breakdown of pigment in the retina (both rods and cones degenerate). It usually starts with night-blindness; as the condition worsens, people may also have difficulty seeing in dim light. Eventually, RP may progress to loss of peripheral vision, which leads to tunnel vision. Color vision may also be affected.
Read more at Guide Dog Association: www.guidedogs.com.au/quality-of-life/common-vision-impairments/retinitis-pigmentosa
The retina in our eyes contains two basic types of light sensors (photoreceptors), rods and cones, which form independent visual systems that are dedicated to special tasks. Rods are primarily used for brightness and motion perception, as well as for night vision.
Actions and activities assigned to or required or expected of a person or group. With respect to accessibility, we have to care for the following three roles:
A computer program that reads the screen to a user. It can be used to surf the Web, write a spreadsheet or document, or just to read pages. It is closely related to voice output.
See also Jaws
A software program that magnifies a portion of the screen, so that it can be more easily viewed. Screen magnifiers are used primarily by individuals with low vision.
Section of the US Rehabilitation Act of 1973 that protects people in federally-funded programs from discrimination on the basis of a disability.
The term commonly used to refer to the United States federal government's recent enactment of regulations regarding equal access to information technology for people with disabilities. Specifically, Section 508 refers to Section 508 of the United States Federal Rehabilitation Act. In 1998, the United States government amended the Rehabilitation Act to add this section.
Section 508 establishes accessibility requirements for any electronic and information technology that is developed, maintained, procured, or used by the United States federal government. It requires that all U.S. government agencies "ensure that federal employees with disabilities have access to and use of information and data that is comparable to the access of those without disabilities." It also requires that federal agencies developing Web sites ensure that citizens with disabilities have equal access to the information on those Web sites.
To be "Section 508 compliant," software, Web sites, documentation, and technical support purchased by the federal government after June 25, 2001, must meet the accessibility standards specified in the Section 508 regulations. These standards, as well as definitions of "electronic and information technology," were issued by the U.S. Access Board (an independent government agency) on December 21, 2000.
Accelerator keys and function keys are further approaches to providing keyboard access. These keys are used to
Currently, Web pages rarely use this feature but for a portal application it is very useful.
With HTML 4.0, the W3C introduced the accessKey attribute that allows the user to access that element by pressing an accelerator key; these are Alt+"Key" combinations.
Convert electronic text to speech, and can be used with a variety of access hardware and software.
See also: Voice Output
A markup language under development by the W3C that will allow Web developers to separate the content of multimedia into distinct files and transmission streams such as, text, images, audio, and video. They can then be sent to the users' computer separately, and then reassembled and displayed as intended.
Manages the list of element on a page that can have the focus (also known as accessibility hierarchy). These active elements are automatically added to the hierarchy by the browser when it parses the source code. By default, elements are added to the hierarchy in the order they appear in the HTML source of the page (source order).
See Tab Chain
See Tab Chain
Images used by blind people to obtain information that sighted people get from looking at pictures. Students learning geography for example would be lost without maps of regions being studied. A blind student without equivalent tactile maps is at an enormous disadvantage relative to sighted peers.
Term used to describe the technique of providing a text alternative that will be the same in both content and function as a non-text object on a Web page, such as an image map.
See also: Descriptions
Tritanopia or blue-yellow blindness is the malfunctioning of the blue cone system. People with tritanopia are dichromats having only one malfunctioning cone system. They confuse blue with green and yellow with violet. So the term blue-green color blindness would be more accurate because the colors blue and yellow are usually not mixed up by tritanopes. Another description of what people having tritanopia perceive is: Longer wavelengths appear as red and the shorter ones as bluish-green.
Two different types can be observed:
This color deficiency is very rare.
Belongs to the receptor-related impairments and is like seeing the world through a small tube. This makes it very hard for people to maintain orientation in their daily lives. Two common causes of tunnel vision are glaucoma and retinitis pigmentosa. In the latter case, tunnel vision may be combined with night-blindness.
In computing, a uniform resource locator or universal resource locator (URL) is a specific character string that constitutes a reference to an Internet resource. A URL is technically a type of uniform resource identifier (URI) but in many technical documents and verbal discussions URL is often used as a synonym for URI. (From Wikipedia, adapted)
Means, as a general term, that all persons regardless of whether they are poor or rich, have disabilities or not, etc. can access something, be it nature, works of art, technology or, more specifically, information technology.
With respect to information technology, "universal access" means that computer applications and Web pages must be universally accessible. In many countries, this is required by legislation, for example, in the United States by Section 508.
Designing for the largest audience possible regardless of disability or ability to speak the native language. This is a process rather than an end in itself.
The URC SIG defines a Universal Remote Console (URC) as "a combination of hardware and software that allows a user to control and view displays of any (compatible) electronic and information technology device or service (or "target") in a way that is accessible and convenient to the user."
The basic idea behind the URC is to define a universal protocol (Alternative Interface Access Protocol = AIAP) that enables any device to command any other device via remote signals. This approach is especially promising for disabled and elderly people but brings hope to all those many people who cannot program video recorders, use copy machines, or handle "intelligent" microwave ovens. With an URC, people can use devices of their choice as remote controls for other devices.
Is a philosophy as well as a process. It is a philosophy that places the person (as opposed to the 'thing') at the center; it is a process that focuses on cognitive factors (such as perception, memory, learning, problem-solving, etc.) as they come into play during peoples' interactions with things. UCD seeks to answer questions about users and their tasks and goals, then use the findings to drive development and design. UCD can improve the usability and usefulness of everything from "everyday things" (D. Norman) to software to information systems to processes – anything with which people interact. As such, User-Centered Design concerns itself with both usefulness and usability.
Visual information is sent from the eye through the optic nerve to the brain. When the optic nerve or those parts of the brain that are used for seeing are damaged, parts of or even the whole vision is lost. Which part of the vision field is missing, depends on the affected neural pathway(s). The eyes may still work normally.
Vision field loss, a neural-related impairment, can have many causes, such as strokes/CVA (cerebral vascular accident), brain tumors, post surgery implications, or head injury (e.g. caused by car accidents).
People with visual impairments range from the totally blind to people who have some difficulty reading small print.
Systems that allow a person to access a computer by entering data and issue commands to the computer with spoken words without using a keyboard or mouse.
Spoken words that are conveyed to the user from the computer.
Technology by which sounds, words or phrases spoken by humans are converted into electrical signals, and these signals are transformed into coding patterns that can be identified by a computer. Based on this identification, the computer usually takes some action.
See World Wide Web
The Web Accessibility Initiative (WAI) pursues, in coordination with organizations around the world, accessibility of the Web through five primary areas of work: technology, guidelines, tools, education and outreach, and research and development. (From homepage)
The Web Content Accessibility Guidelines (WCAG) explain how to make Web content accessible to people with disabilities. The guidelines are intended for all Web content developers (page authors and site designers) and for developers of authoring tools. The primary goal of these guidelines is to promote accessibility. However, following them will also make Web content more available to all users, whatever user agent they are using (e.g., desktop browser, voice browser, mobile phone, automobile-based personal computer, etc.) or constraints they may be operating under (e.g., noisy surroundings, under- or over-illuminated rooms, in a hands-free environment, etc.). Following these guidelines will also help people find information on the Web more quickly. These guidelines do not discourage content developers from using images, video, etc., but rather explain how to make multimedia content more accessible to a wide audience. (From abstract, modified)
The World Wide Web Consortium (W3C) published the Web Content Accessibility Guidelines 1.0 (WCAG 1.0) as a recommendation in May 1999. The updated for version 2.0 has been published in December 2008 and builds on WCAG 1.0. It has the same aim: to explain how to make Web content accessible to people with disabilities and to define target levels of accessibility. Incorporating feedback on WCAG 1.0, the new version 2.0 focuses on guidelines. It attempts to apply guidelines to a wider range of technologies and to use wording that may be understood by a more varied audience. (From abstract)
A Web page or Webpage is a document or information resource that is suitable for the World Wide Web and can be accessed through a Web browser and displayed on a monitor or mobile device according to its markup instructions. This information is usually in HTML or XHTML format, and may provide navigation to other Web pages via clickable hypertext links. Web pages frequently subsume other resources such as style sheets, scripts, and images, which may also be clickable, into their final presentation. Pages can also be designed to make use of applets (subprograms than run inside the page), which often provide motion graphics, interaction, and sound.
Web pages may be retrieved from a local computer or from a remote Web server. The Web server may restrict access only to a private network, e.g. a corporate Intranet, or it may publish pages on the World Wide Web. Web pages are requested and served from Web servers using Hypertext Transfer Protocol (HTTP), which may optionally employ encryption (HTTP Secure, HTTPS) to provide security and privacy for the user of the Web page content
A Website (Web site) is a collection of related Web pages containing images, videos or other digital assets. It is hosted on at least one Web server, accessible via a network such as the Internet or a private local area network. The pages can usually be accessed through an Internet address called uniform resource locator (URL). The base address refers to the site's homepage, the remaining pages have addresses of their own depending on the site structure. The URLs of the pages organize them into a hierarchy, although hyperlinking between them conveys the reader's perceived site structure and guides the reader's navigation of the site. All publicly accessible Websites collectively constitute the World Wide Web. (From Wikipedia, adapted)
A software widget is a generic type of software application comprising portable code intended for one or more different software platforms. The term often implies that either the application, user interface, or both, are light, meaning relatively simple and easy to use, as exemplified by a desk accessory or applet, as opposed to a more complete software package such as a spreadsheet or word processor. Widgets often take the form of on-screen devices (clocks, event countdowns, auction-tickers, stock market tickers, flight arrival information, daily weather etc.).
Consortium that develops interoperable technologies (specifications, guidelines, software, and tools) to lead the Web to its full potential. W3C is a forum for information, commerce, communication, and collective understanding. (From homepage)
XHTML (eXtensible HyperText Markup Language) is a family of XML markup languages that mirror or extend versions of the widely-used hypertext markup language (HTML), the language in which Web pages are written.
While HTML (prior to HTML5) was defined as an application of Standard Generalized Markup Language (SGML), a very flexible markup language framework, XHTML is an application of XML, a more restrictive subset of SGML. Because XHTML documents need to be well-formed, they can be parsed using standard XML parsers – unlike HTML, which requires a lenient HTML-specific parser. (From Wikipedia, adapted)
Extensible Markup Language (XML) is a set of rules for encoding documents in machine-readable form. The design goals of XML emphasize simplicity, generality, and usability over the Internet. It is a textual data format with strong support via Unicode for the languages of the world. Although the design of XML focuses on documents, it is widely used for the representation of arbitrary data structures, for example in Web services. (From Wikipedia, adapted)
XUL is used to define what the user interface will look like for a particular piece of software. For example, XUL is used to define what buttons, scrollbars, text boxes, and other user-interface items will appear, but it is not used to define how those item will look (e.g. what color they are).