functions.php

<?php
   /**
   * @note : Caption content is filtered to use html5 semantic elements figure & figcaption. First found on Zhen Huang's "Reverie".
   * @author : @milohuang
   * @see : http://themefortress.com/reverie/
   * @see : https://github.com/milohuang/reverie/blob/master/lib/clean.php
   * @note : Then we add microdata. Found on Joost Kiens' Gists.
   * @author : @joostkiens
   * @see : https://gist.github.com/JoostKiens/4477366
   * @note : And in the end I add all ARIA role and attributes we need (group, aria-labelledby, aria-describedby). Group role is due to a lack of support of figure semantic.
   * @see : http://www.kloh.ch/craw2013-html5-aria-et-accessibilite-web-479
   * 
   * @param  string $val     Empty
   * @param  array  $attr    Shortcode attributes
   * @param  string $content Shortcode content
   * @return string          Shortcode output
   */
  function ffeeeedd__caption( $output, $attr, $content ) {
    if ( is_feed() ) {
      return $output;
    }
    $defaults = array(
      'id' => '',
      'align' => 'alignnone',
      'width' => '',
      'caption' => ''
    );
    $attr = shortcode_atts( $defaults, $attr );
    if ( 1 > (int) $attr['width'] || empty( $attr['caption'] ) ) {
      return $content;
    }
    $content = str_replace( '<img', '<img id="' . $attr['id'] . '" itemprop="contentURL" aria-labelledby="wp-caption_' . $attr['id'] . '"', $content );
    $attributes = ' class="wp-caption ' . esc_attr( $attr['align'] ) . '"';
    $output = '<figure' . $attributes .' role="group" itemscope itemtype="http://schema.org/ImageObject">';
    $output .= do_shortcode( $content );
    $output .= '<figcaption class="wp-caption-text pt1 " id="wp-caption_' . $attr['id'] . '" aria-describedby="' . $attr['id'] . '" itemprop="description">' . $attr['caption'] . '</figcaption>';
    $output .= '</figure>';
    return $output;
  }
  add_filter( 'img_caption_shortcode', 'ffeeeedd__caption', 10, 3 );