Incorrect ARIA markup for dismissible messages

Problem/Motivation

The Radix theme markup for dismissible Drupal messages (errors, alerts, etc) generates inaccessible code. The Close button is natively focusable, but it is marked as aria-hidden="true". This will make it unusable for users of assistive technology such as screenreaders.

The Bootstrap accessibility JS includes a function to clean this up, but not all sites will be using that plugin.

Steps to reproduce

1. Install a Drupal 7 site with a Radix-based theme that doesn't use the Bootstrap accessibility JS.
2. Take an action that results in a Drupal status message being output to the browser - for example, create and save a node.
3. View the source code of the resulting page.
4. Note the markup of the status message:

<div id="messages">
  <div class="alert alert-success alert-dismissable">
    <button type="button" class="close" data-dismiss="alert" aria-hidden="true">&times;</button>
    Page <em class="placeholder">Test content page</em> has been created.
  </div>
</div>

Proposed resolution

There are several possible ways to mark this up:
Use an aria-label on the button and remove the aria-hidden attribute:

<div id="messages">
  <div class="alert alert-success alert-dismissable">
    <button type="button" class="close" data-dismiss="alert" aria-label="Close">&times;</button>
    Page <em class="placeholder">Test content page</em> has been created.
  </div>
</div>

Do the same thing that the Bootstrap JS does, and add screenreader-only text:

<div id="messages">
  <div class="alert alert-success alert-dismissable">
    <button type="button" class="close" data-dismiss="alert">
      <span aria-hidden="true">&times;</span>
      <span class="sr-only">Close</span>
    </button>
    Page <em class="placeholder">Test content page</em> has been created.
  </div>
</div>

The aria-label approach has one possible drawback, which is that element attributes are not usually translated by things like Google Translate.

Remaining tasks

• Decide on best approach.
• Patch and test.

User interface changes

None.

API changes

None.

Data model changes

None.