Support Content-Type different from text/html

You can use
drupal_set_header('Content-Type: application/xhtml+xml');
in e.g. template.php. This will replace the default text/html header.
Then change the http-equiv from page.tpl.php.

The hard-coded value has been added to avoid issues in the case a theme doesn't set it; for what I remember, the problem is not setting the encoding to UTF-8.
I am looking for the issue reported about that for another report; I will add here the link.

Given the amount of Javascript that application/xhtml+xml breaks, it's safe to bump that to D8. By the time D8 is released, HTML5 will probably have taken off, which will make this feature request moot.

+1

XHTML doesn't seem worth it unless if the browser is actually interpreting the pages as XML and following standards instead of using quirks mode. Drupal appears to be ``half compliant'' which really means no compliance.

Also, jQuery().html() breaks with application/xhtml+xml.

You can use the php str_replace() function to replace the string. See the example below that changes

// replace the content-type tag for Drupal 6
function ThemeName_preprocess_page(&$vars, $hook) {
  $vars['head'] = str_replace(
    '<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />',
    '<meta charset="utf-8" />',
    $vars['head']
  );
}

// replace the content-type tag for Drupal 7
function ThemeName_html_head_alter(&$head_elements) {
  $head_elements['system_meta_content_type']['#attributes'] = array(
    'charset' => 'utf-8'
  );
}

tag

Nowadays IE9 + jquery-1.5 finally seem to work with Content-Type: application/xhtml+xml; charset=utf-8 so hopefully this bug's fix isn't too far away....

Drupal 8 is now serving HTML5 by default.

Presumably WSCCI should make it possible to serve any arbitrary requested mimetype. xhtml is not a specific target we're concerned about, though, as it is basically incompatible with any site that accepts user-entered content.

Marking this won't fix as it's not a primary task, but more of a convenient side-effect.