Need to add namespace attribute to head

Thanks so much, Jeff.

So template.php would look like the below, right? (This is a live site, so I want to be careful here.)

/**
 * Override or insert variables into the html template.
 */
function at_commerce_preprocess_html(&$vars) {

  global $theme_key;

  $theme_name = 'at_commerce';
  $path_to_theme = drupal_get_path('theme', $theme_name);
  
    // Inject the opengraph XML namespace attribute into the the html element
  $vars['html_attributes_array']['xmlns:og'][] = 'http://opengraphprotocol.org/schema/';
}

  // Add a class for the active color scheme
  if (module_exists('color')) {
    $class = check_plain(get_color_scheme_name($theme_key));
    $vars['classes_array'][] = 'color-scheme-' . drupal_html_class($class);
  }

  // Add class for the active theme
  $vars['classes_array'][] = drupal_html_class($theme_key);

  // Add browser and platform classes
  $vars['classes_array'][] = css_browser_selector();

  // Add theme settings classes
  $settings_array = array(
    'body_background',
    'header_layout',
    'menu_bullets',
    'main_menu_alignment',
    'corner_radius_form_input_text',
    'corner_radius_form_input_submit',
  );
  foreach ($settings_array as $setting) {
    $vars['classes_array'][] = at_get_setting($setting);
  }

  // Special case for PIE htc rounded corners, not all themes include this
  if (at_get_setting('ie_corners') == 1) {
    drupal_add_css($path_to_theme . '/css/ie-htc.css', array(
      'group' => CSS_THEME,
      'browsers' => array(
        'IE' => 'lte IE 8',
        '!IE' => FALSE,
        ),
      'preprocess' => FALSE,
      )
    );
  }

  // Custom settings for AT Commerce
  // Content displays
  $show_frontpage_grid = at_get_setting('content_display_grids_frontpage') == 1 ? TRUE : FALSE;
  $show_taxopage_grid = at_get_setting('content_display_grids_taxonomy_pages') == 1 ? TRUE : FALSE;
  if ($show_frontpage_grid == TRUE || $show_taxopage_grid == TRUE) {drupal_add_js($path_to_theme . '/scripts/equalheights.js');}
  if ($show_frontpage_grid == TRUE) {
    $cols_fpg = at_get_setting('content_display_grids_frontpage_colcount');
    $vars['classes_array'][] = $cols_fpg;
    drupal_add_js($path_to_theme . '/scripts/eq.fp.grid.js');
  }
  if ($show_taxopage_grid == TRUE) {
    $cols_tpg = at_get_setting('content_display_grids_taxonomy_pages_colcount');
    $vars['classes_array'][] = $cols_tpg;
    drupal_add_js($path_to_theme . '/scripts/eq.tp.grid.js');
  }

  // Do stuff for the slideshow
  if (at_get_setting('show_slideshow') == 1) {
    // Add some js and css
    drupal_add_css($path_to_theme . '/css/styles.slideshow.css', array(
      'preprocess' => TRUE,
      'group' => CSS_THEME,
      'media' => 'screen',
      'every_page' => TRUE,
      )
    );
    drupal_add_js($path_to_theme . '/scripts/jquery.flexslider-min.js');
    drupal_add_js($path_to_theme . '/scripts/slider.options.js');

    // Add some classes to do evil hiding of elements with CSS...
    if (at_get_setting('show_slideshow_navigation_controls') == 0) {
      $vars['classes_array'][] = 'hide-ss-nav';
    }
    if (at_get_setting('show_slideshow_direction_controls') == 0) {
      $vars['classes_array'][] = 'hide-ss-dir';
    }

    // Write some evil inline CSS in the head, oh er..
    $slideshow_width = check_plain(at_get_setting('slideshow_width'));
    $slideshow_css = '.flexible-slideshow,.flexible-slideshow .article-inner,.flexible-slideshow .article-content,.flexslider {max-width: ' .  $slideshow_width . 'px;}';
    drupal_add_css($slideshow_css, array(
      'group' => CSS_DEFAULT,
      'type' => 'inline',
      )
    );
  }

  // Draw stuff
  drupal_add_js($path_to_theme . '/scripts/draw.js');

}

etc.

Thanks, Jeff - made the change!

One more question, though (and I really appreciate your timely and excellent help with all this):

The attribute seems to be declared only for IE - which might be correct, as far as I know. Below is the beginning of the page source, which is the only place the declaration appears. Is that right?

Thanks again here...

<!DOCTYPE html>
<!--[if IEMobile 7]><html class="iem7" lang="en" dir="ltr" xmlns:og="http://opengraphprotocol.org/schema/"><![endif]-->
<!--[if lte IE 6]><html class="lt-ie9 lt-ie8 lt-ie7" lang="en" dir="ltr" xmlns:og="http://opengraphprotocol.org/schema/"><![endif]-->
<!--[if (IE 7)&(!IEMobile)]><html class="lt-ie9 lt-ie8" lang="en" dir="ltr" xmlns:og="http://opengraphprotocol.org/schema/"><![endif]-->
<!--[if IE 8]><html class="lt-ie9" lang="en" dir="ltr" xmlns:og="http://opengraphprotocol.org/schema/"><![endif]-->
<!--[if (gte IE 9)|(gt IEMobile 7)]><!--><html lang="en" dir="ltr" xmlns:og="http://opengraphprotocol.org/schema/" prefix="content: http://purl.org/rss/1.0/modules/content/ dc: http://purl.org/dc/terms/ foaf: http://xmlns.com/foaf/0.1/ og: http://ogp.me/ns# rdfs: http://www.w3.org/2000/01/rdf-schema# sioc: http://rdfs.org/sioc/ns# sioct: http://rdfs.org/sioc/types# skos: http://www.w3.org/2004/02/skos/core# xsd: http://www.w3.org/2001/XMLSchema#"><!--<![endif]-->
<head>

etc. The declaration appears nowhere else. Again - this could be right - just checking... and sorry to be a pain. Thanks much!

Thanks, Jeff - you're my hero. Issue closed!