Drupal 7 Accessible Pager (Backport from Drupal 8)

pager.tpl.php

<?php
// Place this file your theme's "templates" folder.

/**
 * @file
 * Theme override to display a pager.
 *
 * This is a backport of pager.html.twig from Drupal 8 to Drupal 7, which add
 * accessibility support for WCAG 2.0 section 2.4.9.
 *
 * Available variables:
 * - items: List of pager items.
 *   The list is keyed by the following elements:
 *   - first: Item for the first page; not present on the first page of results.
 *   - previous: Item for the previous page; not present on the first page
 *     of results.
 *   - next: Item for the next page; not present on the last page of results.
 *   - last: Item for the last page; not present on the last page of results.
 *   - pages: List of pages, keyed by page number.
 *   Sub-sub elements:
 *   items.first, items.previous, items.next, items.last, and each item inside
 *   items.pages contain the following elements:
 *   - href: URL with appropriate query parameters for the item.
 *   - attributes: A keyed list of HTML attributes for the item.
 *   - text: The visible text used for the item link, such as "‹ previous"
 *     or "next ›".
 * - current: The page number of the current page.
 * - ellipses: If there are more pages than the quantity allows, then an
 *   ellipsis before or after the listed pages may be present.
 *   - previous: Present if the currently visible list of pages does not start
 *     at the first page.
 *   - next: Present if the visible list of pages ends before the last page.
 *
 * @see template_preprocess_pager()
 */
?>
<?php if (count($items)) : ?>
  <nav class="item-list item-list--pager" role="navigation" aria-labelledby="pagination-heading">
    <h4 id="pagination-heading" class="element-invisible"><?= t('Pagination') ?></h4>
    <ul class="pager">
      <?php // Print first item if we are not on the first page. ?>
      <?php if ($items['first']) : ?>
        <li class="pager-first">
          <a href="<?= $items['first']['href'] ?>" title="<?= t('Go to first page') ?>"<?= $items['first']['attributes'] ?>>
            <span class="element-invisible"><?= t('First page') ?></span>
            <span aria-hidden="true"><?= t('« first') ?></span>
          </a>
        </li>
      <?php endif; ?>
      <?php // Print previous item if we are not on the first page. ?>
      <?php if ($items['previous']) : ?>
        <li class="pager-previous">
          <a href="<?= $items['previous']['href'] ?>" title="<?= t('Go to previous page') ?>" rel="prev"<?= $items['previous']['attributes'] ?>>
            <span class="element-invisible"><?= t('Previous page') ?></span>
            <span aria-hidden="true"><?= t('‹ previous') ?></span>
          </a>
        </li>
      <?php endif; ?>
      <?php // Add an ellipsis if there are further previous pages. ?>
      <?php if ($ellipses['previous']) : ?>
        <li class="pager-ellipsis" role="presentation">&hellip;</li>
      <?php endif; ?>
      <?php // Now generate the actual pager piece. ?>
      <?php foreach($items['pages'] as $key => $item) : ?>
        <li class="pager-item<?= $current == $key ? ' pager-current' : '' ?>">
          <?php if ($current == $key) : ?>
            <?php $title = t('Current page'); ?>
          <?php else : ?>
            <?php $title = t('Go to page @key', array('@key' => $key)); ?>
          <?php endif; ?>
          <a href="<?= $item['href'] ?>" title="<?= $title ?>" class="<?= $current == $key ? ' active' : '' ?>">
            <span class="element-invisible">
              <?= $current == $key ? t('Current page') : t('Page'); ?>
            </span><?= $key ?>
          </a>
        </li>
      <?php endforeach; ?>
      <?php // Add an ellipsis if there are further next pages. ?>
      <?php if ($ellipses['next']) : ?>
        <li class="pager-ellipsis" role="presentation">&hellip;</li>
      <?php endif; ?>
      <?php // Print next item if we are not on the last page. ?>
      <?php if ($items['next']) : ?>
        <li class="pager-next">
          <a href="<?= $items['next']['href'] ?>" title="<?= t('Go to next page') ?>" rel="next"<?= $items['next']['attributes'] ?>>
            <span class="element-invisible"><?= t('Next page') ?></span>
            <span aria-hidden="true"><?= t('next ›') ?></span>
          </a>
        </li>
      <?php endif; ?>
      <?php // Print last item if we are not on the last page. ?>
      <?php if ($items['last']) : ?>
        <li class="pager-last">
          <a href="<?= $items['last']['href'] ?>" title="<?= t('Go to last page') ?>"<?= $items['last']['attributes'] ?>>
            <span class="element-invisible"><?= t('Last page') ?></span>
            <span aria-hidden="true"><?= t('last »') ?></span>
          </a>
        </li>
      <?php endif; ?>
    </ul>
  </nav>
<?php endif; ?>

template.php

<?php
// Paste the code below in your theme's template.php file.

/**
 * Implements hook_preprocess_pager().
 *
 * This is a backport of template_preprocess_pager from Drupal 8.
 */
function template_preprocess_pager(&$variables) {
  $element = $variables['element'];
  $parameters = $variables['parameters'];
  $quantity = $variables['quantity'];
  $route_name = $_GET['q'];
  global $pager_page_array, $pager_total;

  // Nothing to do if there is only one page.
  if ($pager_total[$element] <= 1) {
    return;
  }

  $tags = $variables['tags'];

  // Calculate various markers within this pager piece:
  // Middle is used to "center" pages around the current page.
  $pager_middle = ceil($quantity / 2);
  // current is the page we are currently paged to
  $pager_current = $pager_page_array[$element] + 1;
  // first is the first page listed by this pager piece (re quantity)
  $pager_first = $pager_current - $pager_middle + 1;
  // last is the last page listed by this pager piece (re quantity)
  $pager_last = $pager_current + $quantity - $pager_middle;
  // max is the maximum page number
  $pager_max = $pager_total[$element];
  // End of marker calculations.

  // Prepare for generation loop.
  $i = $pager_first;
  if ($pager_last > $pager_max) {
    // Adjust "center" if at end of query.
    $i = $i + ($pager_max - $pager_last);
    $pager_last = $pager_max;
  }
  if ($i <= 0) {
    // Adjust "center" if at start of query.
    $pager_last = $pager_last + (1 - $i);
    $i = 1;
  }
  // End of generation loop preparation.

  // Create the "first" and "previous" links if we are not on the first page.
  if ($pager_page_array[$element] > 0) {
    $items['first'] = array();
    $options = array(
      'query' => pager_query_add_page($parameters, $element, 0),
    );
    $items['first']['href'] = url($route_name, $options);
    if (isset($tags[0])) {
      $items['first']['text'] = $tags[0];
    }

    $items['previous'] = array();
    $options = array(
      'query' => pager_query_add_page($parameters, $element, $pager_page_array[$element] - 1),
    );
    $items['previous']['href'] = url($route_name, $options);
    if (isset($tags[1])) {
      $items['previous']['text'] = $tags[1];
    }
  }

  if ($i != $pager_max) {
    // Add an ellipsis if there are further previous pages.
    if ($i > 1) {
      $variables['ellipses']['previous'] = TRUE;
    }
    // Now generate the actual pager piece.
    for (; $i <= $pager_last && $i <= $pager_max; $i++) {
      $options = array(
        'query' => pager_query_add_page($parameters, $element, $i - 1),
      );
      $items['pages'][$i]['href'] = url($route_name, $options);
      if ($i == $pager_current) {
        $variables['current'] = $i;
      }
    }
    // Add an ellipsis if there are further next pages.
    if ($i < $pager_max + 1) {
      $variables['ellipses']['next'] = TRUE;
    }
  }

  // Create the "next" and "last" links if we are not on the last page.
  if ($pager_page_array[$element] < ($pager_max - 1)) {
    $items['next'] = array();
    $options = array(
      'query' => pager_query_add_page($parameters, $element, $pager_page_array[$element] + 1),
    );
    $items['next']['href'] = url($route_name, $options);
    if (isset($tags[3])) {
      $items['next']['text'] = $tags[3];
    }

    $items['last'] = array();
    $options = array(
      'query' => pager_query_add_page($parameters, $element, $pager_max - 1),
    );
    $items['last']['href'] = url($route_name, $options);
    if (isset($tags[4])) {
      $items['last']['text'] = $tags[4];
    }
  }

  $variables['items'] = $items;
}

/**
 * Backport of pager_query_add_page() from Drupal 8.
 */
function pager_query_add_page(array $query, $element, $index) {
  global $pager_page_array;

  // Determine the first result to display on the linked page.
  $page_new = pager_load_array($index, $element, $pager_page_array);

  $page = isset($_GET['page']) ? $_GET['page'] : '';
  if ($new_page = implode(',', pager_load_array($page_new[$element], $element, explode(',', $page)))) {
    $query['page'] = $new_page;
  }
  // Merge the query parameters passed to this function with the parameters
  // from the current request. In case of collision, the parameters passed
  // into this function take precedence.
  if ($current_request_query = pager_get_query_parameters()) {
    $query = array_merge($current_request_query, $query);
  }
  return $query;
}

I believe I've not correctly written the functionality to allow proper ability to override the "tags" text. (first, last, next, previous)

 <span aria-hidden="true"><?= t('last »') ?></span>

^ doesn't properly take the $items['last']['text'] into account..

   if (isset($tags[4])) {
      $items['last']['text'] = $tags[4];
    }

I've created an issue on Drupal.org to get this officially backported: https://www.drupal.org/node/2550929