Accessibility: Improve the way Internet Explorer 11 and JAWS announce fieldset legends.

When Internet Explorer encounters a non interactive element with a `tabindex`
attribute, it adds the element to the accessibility tree with a `role=group` and
an accessible name computed from the element. This prevents JAWS from announcing
any fieldset legend within the element.

- removes `tabindex="0"` from the content and the toolbar containers: these tabindex attributes are no longer needed
- removes `aria-label="Main content"` from the content container: not needed
- keeps the media modal focus fallback introduced in [38142] by making the `#wpbody-content` element focusable only when needed

Props stevefaulkner, aardrian.
Fixes #43154.


git-svn-id: https://develop.svn.wordpress.org/trunk@44639 602fd350-edb4-49c9-b593-d223f7449a82

src/js/media/views/modal.js

@@ -137,7 +137,9 @@ Modal = wp.media.View.extend(/** @lends wp.media.view.Modal.prototype */{
 		if ( null !== this.clickedOpenerEl ) {
 			this.clickedOpenerEl.focus();
 		} else {
-			$( '#wpbody-content' ).focus();
+			$( '#wpbody-content' )
+				.attr( 'tabindex', '-1' )
+				.focus();
 		}
 
 		this.propagate('close');

src/wp-admin/admin-header.php

@@ -248,7 +248,7 @@ $current_screen->set_parentage( $parent_file );
 
 ?>
 
-<div id="wpbody-content" aria-label="<?php esc_attr_e( 'Main content' ); ?>" tabindex="0">
+<div id="wpbody-content">
 <?php
 
 $current_screen->render_screen_meta();

src/wp-includes/class-wp-admin-bar.php

@@ -438,7 +438,7 @@ class WP_Admin_Bar {
 			<?php if ( ! is_admin() ) { ?>
 				<a class="screen-reader-shortcut" href="#wp-toolbar" tabindex="1"><?php _e( 'Skip to toolbar' ); ?></a>
 			<?php } ?>
-			<div class="quicklinks" id="wp-toolbar" role="navigation" aria-label="<?php esc_attr_e( 'Toolbar' ); ?>" tabindex="0">
+			<div class="quicklinks" id="wp-toolbar" role="navigation" aria-label="<?php esc_attr_e( 'Toolbar' ); ?>">
 				<?php
 				foreach ( $root->children as $group ) {
 					$this->_render_group( $group );