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Docs: Introduction of accessibility guidelines

docs/documentation/accessibility.md

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+# Accessibility Guidelines
+
+According the [Strategy 2017](http://www.ilias.de/docu/goto_docu_file_5585_download.html)
+ILIAS aims to to be "Usable for Everyone" including users with special needs.
+ILIAS therefore aims to conform with WCAG (Web Content Accessibility Guidelines) 2.0 on
+conformance level A (see [https://www.w3.org/TR/WCAG20/](https://www.w3.org/TR/WCAG20/)
+). WCAGs "cover a wide range of recommendations for making Web content more accessible"
+. They claim that "following these guidelines will make content accessible to a
+wider  range of people with  disabilities, including blindness and low vision, deafness
+and hearing loss, learning disabilities, cognitive limitations, limited movement,
+speech disabilities, photosensitivity and combinations of these". The base guideline
+concerning accessibility in ILIAS is therefore:
+
+- HTML-Output generated by ILIAS **MUST** comply with all WCAG 2.0 level A.
+
+Note: In several scenarios, developers can make not guarantees/statements, whether
+those  guidelines are met by user generated content (such as uploaded videos or files).
+However, developers SHOULD make a best effort, to support users in generating only
+content that meets the listed guidelines.
+
+The WCAG are structured by four principles that provide the foundation for Web
+accessibility: perceivable, operable, understandable, and robust. In the following
+sections we list all WCAG 2.0 level A guidelines we believe to have potential
+consequences on developing ILIAS. Note that they are originate from:
+[WCAG 2.0](https://www.w3.org/TR/WCAG20/). If unsure on how to
+meet some criteria, one can consult [How to Meet WCAG 2.0](https://www.w3.org/WAI/WCAG20/quickref/).
+
+### Principle 1: Perceivable
+Information and user interface components must be presentable to users in ways they can perceive.
+
+#### Guideline 1.1 Text Alternatives
+Provide text alternatives for any non-text content so that it can be changed into other
+  forms people need, such as large print, braille, speech, symbols or simpler language.
+
+- **Non-text Content**: All non-text content that is presented to the user has a text
+alternative that serves the equivalent purpose, except for the situations listed below:
+
+  - **Controls, Input**: If non-text content is a control or accepts user input, then it
+  has a name that describes its purpose.
+
+  - **Time-Based Media**: If non-text content is time-based media, then text alternatives
+  at least provide descriptive identification of the non-text content.
+
+  - **CAPTCHA**: If the purpose of non-text content is to confirm that content is being
+  accessed by a person rather than a computer, then text alternatives that identify and
+  describe the purpose of the non-text content are provided, and alternative forms of
+  CAPTCHA using output modes for different types of sensory perception are provided to
+  accommodate different disabilities.
+
+  - **Decoration, Formatting, Invisible**: If non-text content is pure decoration, is used
+  only for visual formatting, or is not presented to users, then it is implemented in a
+  way that it can be ignored by assistive technology. Note: According to the WAI,
+  decorative images don’t add information to the content of a page. For example, the
+  information provided by the image  might already be given using adjacent text, or the
+  image might be included to make the website more visually attractive
+  [https://www.w3.org/WAI/tutorials/images/decorative/](https://www.w3.org/WAI/tutorials/images/decorative/).
+
+#### Guideline 1.2 Time-based Media
+Provide alternatives for time-based media.
+
+- **Captions (Prerecorded)**: Captions are provided for all prerecorded audio content in
+synchronized media, except when the media is a media alternative for text and is
+clearly labeled as such. (Level A)
+
+#### Guideline 1.3 Adaptable
+Create content that can be presented in different ways (for example simpler layout)
+without losing information or structure.
+
+- **Info and Relationships**: Information, structure, and relationships conveyed through
+presentation can be programmatically determined or are available in text.
+
+- **Meaningful Sequence**: When the sequence in which content is presented affects its
+meaning, a correct reading sequence can be programmatically determined.
+
+- **Sensory Characteristics**: Instructions provided for understanding and operating
+content do not rely solely on sensory characteristics of components such as shape,
+size, visual location, orientation, or sound.
+
+
+#### Guideline 1.4 Distinguishable
+Make it easier for users to see and hear content including separating foreground from background.
+
+- **Use of Color**: Color is not used as the only visual means of conveying information,
+indicating an action, prompting a response, or distinguishing a visual element.
+
+- **Audio Control**: If any audio on a Web page plays automatically for more than 3
+seconds, either a mechanism is available to pause or stop the audio, or a mechanism is
+available to control audio volume independently from the overall system volume level.
+
+
+### Principle 2: Operable
+User interface components and navigation must be operable.
+
+#### Guideline 2.1 Keyboard Accessible
+Make all functionality available from a keyboard.
+
+- **Keyboard**: All functionality of the content is operable through a keyboard interface
+without requiring specific timings for individual keystrokes, except where the
+underlying function requires input that depends on the path of the user's movement and
+not just the endpoints. This exception relates to the underlying function, not the
+input technique. For example, if using handwriting to enter text, the input technique
+(handwriting) requires path-dependent input but the underlying function (text input)
+does not.  This does not forbid and should not discourage providing mouse input or
+other input methods in addition to keyboard operation.
+
+- **No Keyboard Trap**: If keyboard focus can be moved to a component of the page using a
+keyboard interface, then focus can be moved away from that component using only a
+keyboard interface, and, if it requires more than unmodified arrow or tab keys or other
+standard exit methods, the user is advised of the method for moving focus away. Note:
+Since any content that does not meet this success criterion can interfere with a
+user's ability to use the whole page, all content on the Web page (whether it is used
+to meet other success criteria or not) must meet this success criterion.
+
+
+#### Guideline 2.2 Enough Time
+Provide users enough time to read and use content.
+
+- **Timing Adjustable**: For each time limit that is set by the content, **at least one** of the following is true:
+
+  - **Turn off**: The user is allowed to turn off the time limit before encountering it; or
+
+  - **Adjust**: The user is allowed to adjust the time limit before encountering it over a
+  wide range that is at least ten times the length of the default setting; or
+
+  - **Extend**: The user is warned before time expires and given at least 20 seconds to
+  extend the time limit with a simple action (for example, "press the space bar"), and
+  the user is allowed to extend the time limit at least ten times; or
+
+  - **Real-time Exception**: The time limit is a required part of a real-time event (for
+  example, an auction), and no alternative to the time limit is possible; or
+
+  - **Essential Exception**: The time limit is essential and extending it would invalidate
+  the activity; or
+
+  - **20 Hour Exception**: The time limit is longer than 20 hours.
+
+- **Pause, Stop, Hide**: For moving, blinking, scrolling, or auto-updating information,
+**all of the following** are true:
+
+  - **Moving, blinking, scrolling**: For any moving, blinking or scrolling information that
+   (1) starts automatically, (2) lasts more than five seconds, and (3) is presented in
+   parallel with other content, there is a mechanism for the user to pause, stop, or
+   hide it unless the movement, blinking, or scrolling is part of an activity where it
+   is essential; and
+
+  - **Auto-updating**: For any auto-updating information that (1) starts automatically and
+  (2) is presented in parallel with other content, there is a mechanism for the user to
+   pause, stop, or hide it or to control the frequency of the update unless the
+   auto-updating is part of an activity where it is essential.
+
+#### Guideline 2.3 Seizures
+Do not design content in a way that is known to cause seizures.
+
+- **Three Flashes or Below Threshold**: Web pages do not contain anything that flashes more
+  than three times in any one second period, or the flash is below the general flash
+  and red flash thresholds.
+
+#### Guideline 2.4 Navigable
+Provide ways to help users navigate, find content, and determine where they are.
+
+- **Bypass Blocks**: A mechanism is available to bypass blocks of content that are repeated
+  on multiple Web pages.
+
+- **Page Titled**: Web pages have titles that describe topic or purpose.
+
+- **Focus Order**: If a Web page can be navigated sequentially and the navigation sequences
+ affect meaning or operation, focusable components receive focus in an order that
+ preserves meaning and operability.
+
+- **Link Purpose (In Context)**: The purpose of each link can be determined from the link
+text alone or from the link text together with its programmatically determined link
+context, except where the purpose of the link would be ambiguous to users in general.
+
+### Principle 3: Understandable
+Information and the operation of user interface must be understandable.
+
+#### Guideline 3.1 Readable
+Make text content readable and understandable.
+
+- **Language of Page**: The default human language of each Web page can be programmatically determined.
+
+#### Guideline 3.2 Predictable
+Make Web pages appear and operate in predictable ways.
+
+- **On Focus**: When any component receives focus, it does not initiate a change of context.
+
+- **On Input**: Changing the setting of any user interface component does not automatically
+ cause a change of context unless the user has been advised of the behavior before
+ using the component.
+
+#### Guideline 3.3 Input Assistance
+Help users avoid and correct mistakes.
+
+- **Error Identification**: If an input error is automatically detected, the item that is
+in error is identified and the error is described to the user in text.
+
+- **Labels or Instructions**: Labels or instructions are provided when content requires user input.
+
+
+### Principle 4: Robust
+Content must be robust enough that it can be interpreted reliably by a wide variety of
+user agents, including assistive technologies.
+
+#### Guideline 4.1 Compatible
+Maximize compatibility with current and future user agents, including assistive technologies.
+
+- **Parsing**: In content implemented using markup languages, elements have complete start
+and end tags, elements are nested according to their specifications, elements do not
+contain duplicate attributes, and any IDs are unique, except where the specifications
+allow these features. Start and end tags that are missing a critical character in their
+ formation, such as a closing angle bracket or a mismatched attribute value quotation mark are not complete.
+
+- **Name, Role, Value**: For all user interface components (including but not limited to:
+form elements, links and components generated by scripts), the name and role can be
+programmatically determined; states, properties, and values that can be set by the user
+can be programmatically set; and notification of changes to these items is available
+to user agents, including assistive technologies. This success criterion is primarily
+for Web authors who develop or script their own user interface components. For
+example, standard HTML controls already meet this success criterion when used according
+to specification.
+