Request: Only Replace Local URLs

Suggestion: instead of filtering using the wp_make_link_relative function, perhaps you should add an intermediary filter function that first tests that links share the home url… I think that’s the only time relative links make sense anyway. This seems to work:

Note: I used an anonymous function that uses $home_url, to avoid problems when home_url itself was being filtered.

$home_url = home_url();
$filter_fn = function($link) use ($home_url) {
  if ( strpos($link, $home_url) === 0 ) {
    return wp_make_link_relative( $link );
  } else {
    return $link;
  }
};

foreach ( $filters as $filter ) {
  add_filter( $filter, $filter_fn);
}

http://wordpress.org/plugins/relative-url/