Initial a11y scanning CI template

MVC template that can be included in a gitlab-ci.yml file
Accompanying first set of docs, modeled on Browser Performance

changelogs/unreleased/a11y-ci-template.yml

+---
+title: Initial a11y scanning CI template
+merge_request: 25144
+author:
+type: added

doc/user/project/merge_requests/accessibility_testing.md

+---
+type: reference, howto
+---
+
+# Accessibility Testing
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/25144) in [GitLab Core](https://about.gitlab.com/pricing/) 12.8.
+
+If your application offers a web interface and you are using
+[GitLab CI/CD](../../../ci/README.md), you can quickly determine the accessibility
+impact of pending code changes.
+
+## Overview
+
+GitLab uses [pa11y](https://pa11y.org/), a free and open source tool for
+measuring the accessibility of web sites, and has built a simple
+[CI template](https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/ci/templates/Verify/Accessibility.gitlab-ci.yml)
+which outputs the results in a file called `accessibility`. This outputs
+the accessibility violations, warnings, and notices for each page that is analyzed.
+
+## Configuring Accessiblity Testing
+
+This example shows how to run [pa11y](https://pa11y.org/)
+on your code by using GitLab CI/CD with a plain node Docker image.
+
+For GitLab 12.8 and later, to define the `performance` job, you must
+[include](../../../ci/yaml/README.md#includetemplate) the
+[`Accessibility.gitlab-ci.yml` template](https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/ci/templates/Verify/Accessibility.gitlab-ci.yml)
+that's provided as a part of your GitLab installation.
+For GitLab versions earlier than 12.8, you can copy and use the job as
+defined in that template.
+
+CAUTION: **Caution:**
+The job definition provided by the template does not support Kubernetes yet.
+
+Add the following to your `.gitlab-ci.yml` file:
+
+```yaml
+include:
+  template: Verify/Accessibility.gitlab-ci.yml
+
+a11y:
+  variables:
+    a11y_urls: https://example.com https://example.com/another-page
+```
+
+The above example will create a `a11y` job in your CI/CD pipeline and will run
+Pa11y against the webpage you defined in `a11y_urls` to build a report.
+
+The full HTML Pa11y report will be saved as an artifact that can be viewed directly in your browser.
+
+It is not yet possible to pass configurations into Pa11y via CI configuration. To change anything,
+copy the template to your CI file and make the desired edits.

ee/spec/lib/ee/gitlab/template/gitlab_ci_yml_template_spec.rb

@@ -16,6 +16,7 @@ describe Gitlab::Template::GitlabCiYmlTemplate do
 
     it 'finds the Verify templates' do
       expect(templates).to include('Browser-Performance')
+      expect(templates).to include('Accessibility')
     end
   end
 end

lib/gitlab/ci/templates/Verify/Accessibility.gitlab-ci.yml

+# Read more about the feature here: https://docs.gitlab.com/ee/user/project/merge_requests/accessibility_testing.html
+
+stages:
+  - build
+  - test
+  - deploy
+  - accessibility
+
+a11y:
+  stage: accessibility
+  image: node
+  script:
+    - wget -q -O - https://dl-ssl.google.com/linux/linux_signing_key.pub | apt-key add - && \
+    - echo "deb http://dl.google.com/linux/chrome/deb/ stable main" > /etc/apt/sources.list.d/google.list && \
+    - apt-get update && \
+    - apt-get install -y google-chrome-stable && \
+    - rm -rf /var/lib/apt/lists/*
+    - npm install pa11y@5.3.0 pa11y-reporter-html@1.0.0
+    - 'echo { \"chromeLaunchConfig\": { \"args\": [\"--no-sandbox\"] }, \"includeWarnings\": true, \"reporter\": \"html\" } > pa11y.json'
+    - './node_modules/.bin/pa11y $a11y_urls > accessibility.html'
+  allow_failure: true
+  artifacts:
+    when: always
+    expose_as: 'accessibility'
+    paths: ['accessibility.html']
+  rules:
+    - if: $a11y_urls