accessibility-checker-engine is a rules-based engine for detecting issues in the Document Object Model (DOM) of web applications and content.
- The engine is used by the IBM Equal Access Accessibility Checker suite of tools.
- The rules and engine are written in JavaScript and can be injected directly into web pages and applications.
Install accessibility-checker-engine in a Node environment to inject into a browser environment:
$ npm install --save-dev accessibility-checker-engine
Use a CDN to access the engine in a browser environment:
<script src="https://unpkg.com/accessibility-checker-engine@latest/ace.js"></script>
See CodeSandbox Demo for a more complete example.
const checker = new ace.Checker();
const report = await checker.check(document, ["IBM_Accessibility"]);
The most important entry point API is the check method of ace.Checker object. You can use a callback or Promise mechanism to retrieve the accessibility results for further processing in your javascript or NodeJS program.
const checker = new ace.Checker();
checker.check(doc, ["IBM_Accessibility"])
.then(function(report) {
// process accessibility report here
}); -
doc- can be one of:- a Document Object Model (
DOM) object representing an HTML document which is usually available in a browser environment asdocument - a
DOMelement representing a fragment HTML which can be retrieved from aDOMby matching against one or more selectors.
- a Document Object Model (
-
["IBM_Accessibility"]- apply IBM accessibility ruleset. -
report- accessibility results contains identified accessibility issues and their descriptions from the givendoc, and a summary of the issues. The report is in JSON format (see details).
- Rules are based on the IBM Accessibility requirements, which is a unified set of WCAG, EN 301 549, and US 508 standards.
- Rules are harmonized with the open rules published by the W3C ACT-Rules Community group as reported in the IBM Equal Access Accessibility Checker ACT implementation report.
- Rule sets, such as
IBM Accessibility v7.2,WCAG 2.2 (A & AA),WCAG 2.1 (A & AA), andWCAG 2.0 (A & AA)and mappings of the rules to the standards (Requirements), Rule IDs, the individual failure messages, and links to the Help files are published at Checker rule sets. - Mappings of the rules are defined in the individual rule_ID_name.ts files.
The accessibility report is in JSON format, and contains information about the identified accessibility issues and their descriptions.
{
report: {
scanID: "18504e0c-fcaa-4a78-a07c-4f96e433f3e7",
toolID: "@ibma/aat-v2.0.6",
// Label passed to getCompliance
label: "MyTestLabel",
// Number of rules executed
numExecuted: 137,
nls: {
// Mapping of result.ruleId, result.reasonId to get a tokenized string for the result. Message args are result.messageArgs
"WCAG20_Html_HasLang": {
"Pass_0": "Page language detected as {0}"
},
// ...
},
summary: {
URL: "https://www.ibm.com/en-US/",
counts: {
violation: 1,
potentialviolation: 0,
recommendation: 0,
potentialrecommendation: 0,
manual: 0,
pass: 136,
ignored: 0
},
scanTime: 29,
ruleArchive: "September 2019 Deployment (2019SeptDeploy)",
policies: [
"IBM_Accessibility"
],
reportLevels: [
"violation",
"potentialviolation",
"recommendation",
"potentialrecommendation",
"manual"
],
startScan: 1470103006149
},
results: [
{
// Which rule triggered?
"ruleId": "WCAG20_Html_HasLang",
// In what way did the rule trigger?
"reasonId": "Pass_0",
"value": [
// Is this rule based on a VIOLATION, RECOMMENDATION or INFORMATION
"VIOLATION",
// PASS, FAIL, POTENTIAL, or MANUAL
"PASS"
],
"path": {
// xpath
"dom": "/html[1]",
// path of ARIA roles
"aria": "/document[1]"
},
"ruleTime": 0,
// Generated message
"message": "Page language detected as en",
// Arguments to the message
"messageArgs": [
"en"
],
"apiArgs": [],
// Bounding box of the element
"bounds": {
"left": 0,
"top": 0,
"height": 143,
"width": 800
},
// HTML snippet of the element
"snippet": "<html lang=\"en\">",
// What category is this rule?
"category": "Accessibility",
// Was this issue ignored due to a baseline?
"ignored": false,
// Summary of the value: violation, potentialviolation, recommendation, potentialrecommendation, manual, pass
"level": "pass"
},
// ...
]
}
}This section provides 'AS-IS' code examples, snippets, or logic. The users are expected to make changes according to their environments.
You can use the wrapper method checkDemo in ace object, which is specifically created for checking accessibility in a browser developer tool. The checkDemo method outputs both raw accessibility results in JSON format, and the results sorted by elements identified by their xPath. Following are the example steps to use ace.checkDemo() to display the results in a Chrome developer tool:
- Navigate to a page or type the url to the page in Chrome browser
- Open the developer tool in Chrome browser: click
Customize and Control Google Chromebutton, selectMore Tools, then selectDeveloper Tool - Select
Consoletab to show command prompt - Open
ace.jsselect and copy all the content - Paste the content you copied to the command prompt in the developer tool, then press
Enter - Type in the command prompt:
ace.checkDemo(), thenEnter
You can view the accessibility report for the page:
The following code snippet demonstrates how to use ACE to test a web page for accessibility in an embedded Chrome environment (puppeteer). See accessibility-checker for a more complete tool for this environment.
(async () => {
const chromeLauncher = require('chrome-launcher');
const axios = require('axios');
const puppeteer = require('puppeteer');
// Initialize a Chrome instance
const chrome = await chromeLauncher.launch({
//chromeFlags: ['--headless'],
logLevel: 'info',
output: 'json'
});
const response = await axios.get(`http://localhost:${chrome.port}/json/version`);
const { webSocketDebuggerUrl } = response.data;
// Connect puppeteer to the chrome instance using the endpoint
const browser = await puppeteer.connect({ browserWSEndpoint: webSocketDebuggerUrl });
//get the page
const [page] = await browser.pages();
// inject the ace.js into the page when domcontentloaded event is fired, assuming the ace.js is in the same folder
await page.goto('http://localhost:3000', { waitUtil: 'domcontentloaded' };
await page.addScriptTag({ path: path.join(__dirname, 'ace.js') });
//invoke the ace to evaluate the page for accessibility
await page.evaluate(() => {
const checker = new ace.Checker();
checker.check(document, ["IBM_Accessibility"])
.then(function (report) {
for (let idx = 0; idx < report.results.length; ++idx) {
//process the report
}
});
});
})();You can use the accessibility-checker-extension for Chrome, Edge, or Firefox. The browser extensions integrate the accessibility web engine (ace.js) and formatted results into the browser developer tool to view the accessibility issues and the locations of violating components. For more information and instructions, please view accessibility-checker-extensions.
You can use the karma-accessibility-checker to integrate accessibility web engine into Karma or Selenium test framework. For more information and instructions, please view karma-accessibility-checker.
If you think you've found a bug, have questions or suggestions, please report the bug in GitHub Issues.
This software includes material copied from or derived from the open ACT-Rules Community. Copyright © 2022 W3C® (MIT, ERCIM, Keio, Beihang).