[Docs: A11y] Add accessibility docs (#9519)

* [Docs: A11y] Add accessibility docs

* Fix the link

* Replace link image

* Tweak style

_data/nav_docs.yml

@@ -52,6 +52,8 @@
       title: Higher-Order Components
     - id: integrating-with-other-libraries
       title: Integrating with Other Libraries
+    - id: accessibility
+      title: Accessibility
 - title: Reference
   items:
     - id: react-api

docs/accessibility.md

@@ -0,0 +1,278 @@
+---
+id: accessibility
+title: Accessibility
+permalink: docs/accessibility.html
+redirect_from:
+  - "docs/index.html"
+---
+
+## Why Accessibility?
+
+Web accessibility (also referred to as [**a11y**](https://en.wiktionary.org/wiki/a11y)) is the design and creation of websites that can be used by everyone. Accessibility support is necessary to allow assistive technology to interpret web pages.
+
+React fully supports building accessible websites, often by using standard HTML techniques.
+
+## Standards and Guidelines
+
+### WCAG
+
+The [Web Content Accessibility Guidelines](https://www.w3.org/WAI/intro/wcag) provides guidelines for creating accessible web sites.
+
+The following WCAG checklists provide an overview:
+
+- [WCAG checklist from Wuhcag](https://www.wuhcag.com/wcag-checklist/)
+- [WCAG checklist from WebAIM](http://webaim.org/standards/wcag/checklist)
+- [Checklist from The A11Y Project](http://a11yproject.com/checklist.html)
+
+### WAI-ARIA
+
+The [Web Accessibility Initiative - Accessible Rich Internet Applications](https://www.w3.org/WAI/intro/aria) document contains techniques for building fully accessible JavaScript widgets.
+
+Note that all `aria-*` HTML attributes are fully supported in JSX. Whereas most DOM properties and attributes in React are camelCased, these attributes should be lowercased:
+
+```javascript{3,4}
+<input
+  type="text" 
+  aria-label={labelText}
+  aria-required="true"
+  onChange={onchangeHandler}
+  value={inputValue}
+  name="name"
+/>
+```
+
+## Accessible Forms
+
+### Labeling
+Every HTML form control, such as `<input>` and `<textarea>`, needs to be labeled accessibly. We need to provide descriptive labels that are also exposed to screen readers.
+
+The following resources show us how to do this:
+
+- [The W3C shows us how to label elements](https://www.w3.org/WAI/tutorials/forms/labels/)
+- [WebAIM shows us how to label elements](http://webaim.org/techniques/forms/controls)
+- [The Paciello Group explains accessible names](https://www.paciellogroup.com/blog/2017/04/what-is-an-accessible-name/)
+
+Although these standard HTML practices can be directly used in React, note that the `for` attribute is written as `htmlFor` in JSX:
+
+```javascript{1}
+<label htmlFor="namedInput">Name:</label>
+<input id="namedInput" type="text" name="name"/>
+```
+
+### Notifying the user of errors
+
+Error situations need to be understood by all users. The following link show us how to expose error texts to screen readers as well:
+
+- [The W3C demonstrates user notifications](https://www.w3.org/WAI/tutorials/forms/notifications/)
+- [WebAIM looks at form validation](http://webaim.org/techniques/formvalidation/)
+
+## Focus Control
+
+Ensure that your web application can be fully operated with the keyboard only: 
+
+- [WebAIM talks about keyboard accessibility](http://webaim.org/techniques/keyboard/)
+
+### Keyboard focus and focus outline
+
+Keyboard focus refers to the current element in the DOM that is selected to accept input from the keyboard. We see it everywhere as a focus outline similar to the that shown in the following image:
+
+<img src="/react/img/docs/keyboard-focus.png" alt="Blue keyboard focus outline around a selected link." />
+
+Only ever use CSS that removes this outline, for example by setting `outline: 0`, if you are replacing it with another focus outline implementation.
+
+### Mechanisms to skip to desired content
+
+Provide a mechanism to allow users to skip past navigation sections in your application as this assists and speeds up keyboard navigation.
+
+Skiplinks or Skip Navigation Links are hidden navigation links that only become visible when keyboard users interact with the page. They are very easy to implement with 
+internal page anchors and some styling:
+
+- [WebAIM - Skip Navigation Links](http://webaim.org/techniques/skipnav/)
+
+Also use landmark elements and roles, such as `<main>` and `<aside>`, to demarcate page regions as assistive technology allow the user to quickly navigate to these sections.
+
+Read more about the use of these elements to enhance accessibility here:
+
+- [Deque University - HTML 5 and ARIA Landmarks](https://dequeuniversity.com/assets/html/jquery-summit/html5/slides/landmarks.html)
+
+### Programmatically managing focus
+
+Our React applications continuously modify the HTML DOM during runtime, sometimes leading to keyboard focus being lost or set to an unexpected element. In order to repair this, 
+we need to programmatically nudge the keyboard focus in the right direction. For example, by resetting keyboard focus to a button that opened a modal window after that modal window is closed.
+
+The Mozilla Developer Network takes a look at this and describes how we can build [keyboard-navigable JavaScript widgets](https://developer.mozilla.org/en-US/docs/Web/Accessibility/Keyboard-navigable_JavaScript_widgets).
+
+To set focus in React, we can use [Refs to Components](refs-and-the-dom.html).
+
+Using this, we first create a ref to an element in the JSX of a component class:
+
+```javascript{2-3,7}
+render() {
+  // Use the `ref` callback to store a reference to the text input DOM
+  // element in an instance field (for example, this.textInput).
+  return (
+    <input
+      type="text"
+      ref={(input) => { this.textInput = input; }} />
+  );
+}
+```
+
+Then we can focus it elsewhere in our component when needed:
+
+ ```javascript
+ focus() {
+   // Explicitly focus the text input using the raw DOM API
+   this.textInput.focus();
+ }
+ ```
+
+A great focus management example is the [react-aria-modal](https://github.com/davidtheclark/react-aria-modal). This is a relatively rare example of a fully accessible modal window. Not only does it set initial focus on 
+the cancel button (preventing the keyboard user from accidentally activating the success action) and trap keyboard focus inside the modal, it also resets focus back to the element that 
+initially triggered the modal.
+
+>Note:
+
+>While this is a very important accessibility feature, it is also a technique that should be used judiciously. Use it to repair the keyboard focus flow when it is disturbed, not to try and anticipate how
+>users want to use applications.
+
+## More Complex Widgets
+
+A more complex user experience should not mean a less accessible one. Whereas accessibility is most easily achieved by coding as close to HTML as possible,
+even the most complex widget can be coded accessibly.
+
+Here we require knowledge of [ARIA Roles](https://www.w3.org/TR/wai-aria/roles) as well as [ARIA States and Properties](https://www.w3.org/TR/wai-aria/states_and_properties). 
+These are toolboxes filled with HTML attributes that are fully supported in JSX and enable us to construct fully accessible, highly functional React components.
+
+Each type of widget has a specific design pattern and is expected to function in a certain way by users and user agents alike:
+
+- [WAI-ARIA Authoring Practices - Design Patterns and Widgets](https://www.w3.org/TR/wai-aria-practices/#aria_ex)
+- [Heydon Pickering - ARIA Examples](http://heydonworks.com/practical_aria_examples/)
+- [Inclusive Components](https://inclusive-components.design/)
+
+## Other Points for Consideration
+
+### Setting the language
+
+Indicate the human language of page texts as screen reader software uses this to select the correct voice settings:
+
+- [WebAIM - Document Language](http://webaim.org/techniques/screenreader/#language)
+
+### Setting the document title
+
+Set the document `<title>` to correctly describe the current page content as this ensures that the user remains aware of the current page context:
+
+- [WCAG - Understanding the Document Title Requirement](https://www.w3.org/TR/UNDERSTANDING-WCAG20/navigation-mechanisms-title.html)
+
+We can set this in React using the [React Document Title Component](https://github.com/gaearon/react-document-title).
+
+### Color contrast
+
+Ensure that all readable text on your website has sufficient color contrast to remain maximally readable by users with low vision:
+
+- [WCAG - Understanding the Color Contrast Requirement](https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html)
+- [Everything About Color Contrast And Why You Should Rethink It](https://www.smashingmagazine.com/2014/10/color-contrast-tips-and-tools-for-accessibility/)
+- [A11yProject - What is Color Contrast](http://a11yproject.com/posts/what-is-color-contrast/)
+
+It can be tedious to manually calculate the proper color combinations for all cases in your website so instead, you can [calculate an entire accessible color palette with Colorable](http://jxnblk.com/colorable/).
+
+Both the aXe and WAVE tools mentioned below also include color contrast tests and will report on contrast errors.
+
+If you want to extend your contrast testing abilities you can use these tools:
+
+- [WebAIM - Color Contrast Checker](http://webaim.org/resources/contrastchecker/)
+- [The Paciello Group - Color Contrast Analyzer](https://www.paciellogroup.com/resources/contrastanalyser/)
+
+## Development and Testing Tools
+
+There are a number of tools we can use to assist in the creation of accessible web applications.
+
+### The keyboard
+
+By far the easiest and also one of the most important checks is to test if your entire website can be reached and used with the keyboard alone. Do this by:
+
+1. Plugging out your mouse.
+1. Using `Tab` and `Shift+Tab` to browse.
+1. Using `Enter` to activate elements.
+1. Where required, using your keyboard arrow keys to interact with some elements, such as menus and dropdowns. 
+
+### Development assistance
+
+We can check some accessibility features directly in our JSX code. Often intellisense checks are already provided in JSX aware IDE's for the ARIA roles, states and properties. We also 
+have access to the following tool:
+
+#### eslint-plugin-jsx-a11y
+
+The [eslint-plugin-jsx-a11y](https://github.com/evcohen/eslint-plugin-jsx-a11y) plugin for ESLint provides AST linting feedback regarding accessibility issues in your JSX. Many
+IDE's allow you to integrate these findings directly into code analysis and source code windows.
+
+[Create React App](https://github.com/facebookincubator/create-react-app) has this plugin with a subset of rules activated. If you want to enable even more accessibility rules, 
+you can create an `.eslintrc` file in the root of your project with this content:
+                                                                                                      
+  ```json
+  {
+    "extends": ["react-app", "plugin:jsx-a11y/recommended"],
+    "plugins": ["jsx-a11y"]
+  }
+  ```
+
+### Testing accessibility in the browser
+
+A number of tools exist that can run accessibility audits on web pages in your browser. Please use them in combination with other accessibility checks mentioned here as they can only
+test the technical accessibility of your HTML.
+
+#### aXe, aXe-core and react-axe
+
+Deque Systems offers [aXe-core](https://www.deque.com/products/axe-core/) for automated and end-to-end accessibility tests of your applications. This module includes integrations for Selenium.
+
+[The Accessibility Engine](https://www.deque.com/products/axe/) or aXe, is an accessibility inspector browser extension built on `aXe-core`.
+
+You can also use the [react-axe](https://github.com/dylanb/react-axe) module to report these accessibility findings directly to the console while developing and debugging.
+
+#### WebAIM WAVE
+
+The [Web Accessibility Evaluation Tool](http://wave.webaim.org/extension/) is another accessibility browser extension.
+
+#### Accessibility inspectors and the Accessibility Tree
+
+[The Accessibility Tree](https://www.paciellogroup.com/blog/2015/01/the-browser-accessibility-tree/) is a subset of the DOM tree that contains accessible objects for every DOM element that should be exposed 
+to assistive technology, such as screen readers.
+
+In some browsers we can easily view the accessibility information for each element in the accessibility tree:
+
+- [Activate the Accessibility Inspector in Chrome](https://gist.github.com/marcysutton/0a42f815878c159517a55e6652e3b23a)
+- [Using the Accessibility Inspector in OS X Safari](https://developer.apple.com/library/content/documentation/Accessibility/Conceptual/AccessibilityMacOSX/OSXAXTestingApps.html)
+
+### Screen readers
+
+Testing with a screen reader should form part of your accessibility tests.
+
+Please note that browser / screen reader combinations matter. It is recommended that you test your application in the browser best suited to your screen reader of choice.
+
+#### NVDA in FireFox
+
+[NonVisual Desktop Access](https://www.nvaccess.org/) or NVDA is an open source Windows screen reader that is widely used.
+
+Refer to the following guides on how to best use NVDA:
+
+- [WebAIM - Using NVDA to Evaluate Web Accessibility](http://webaim.org/articles/nvda/)
+- [Deque - NVDA Keyboard Shortcuts](https://dequeuniversity.com/screenreaders/nvda-keyboard-shortcuts)
+
+#### VoiceOver in Safari
+
+VoiceOver is an integrated screen reader on Apple devices. 
+
+Refer to the following guides on how activate and use VoiceOver:
+
+- [WebAIM - Using VoiceOver to Evaluate Web Accessibility](http://webaim.org/articles/voiceover/)
+- [Deque - VoiceOver for OSX Keyboard Shortcuts](https://dequeuniversity.com/screenreaders/voiceover-keyboard-shortcuts)
+- [Deque - VoiceOver for iOS Shortcuts](https://dequeuniversity.com/screenreaders/voiceover-ios-shortcuts)
+
+#### JAWS in Internet Explorer
+
+[Job Access With Speech](http://www.freedomscientific.com/Products/Blindness/JAWS) or JAWS, is a prolifically used screen reader on Windows. 
+
+Refer to the following guides on how to best use JAWS:
+
+- [WebAIM - Using JAWS to Evaluate Web Accessibility](http://webaim.org/articles/jaws/)
+- [Deque - JAWS Keyboard Shortcuts](https://dequeuniversity.com/screenreaders/jaws-keyboard-shortcuts)