// Configure TinyMCE editor not to strip out schema attributes
//      Configuration copied from https://snipt.net/jamesw/prevent-tinymce-from-stripping-schemaorg-attributes-in-wordpress/
function hm_change_mce_options($in) {
    if(!empty($in['extended_valid_elements']))
        $in['extended_valid_elements'] .= ',';

    $in['extended_valid_elements'] .= '@[id|class|style|title|itemscope|itemtype|itemprop|datetime|rel],div,dl,ul,dt,dd,li,span,a|rev|charset|href|lang|tabindex|accesskey|type|name|href|target|title|class|onfocus|onblur]';

    return $in;
}
add_filter('tiny_mce_before_init', 'hm_change_mce_options');