Use schema.org HTML in TinyMCE Editor

I want to use HTML (schema.org) in my TinyMCE Editor. I use the following script in my custom.php

// Prevent TinyMCE from stripping out schema.org metadata
function schema_TinyMCE_init($in)
{
    /**
     *   Edit extended_valid_elements as needed. For syntax, see
     *   http://www.tinymce.com/wiki.php/Configuration:valid_elements
     *
     *   NOTE: Adding an element to extended_valid_elements will cause TinyMCE to ignore
     *   default attributes for that element.
     *   Eg. a[title] would remove href unless included in new rule: a[title|href]
     */
    if(!empty($in['extended_valid_elements']))
        $in['extended_valid_elements'] .= ',';

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

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

The code works for some elements like <span >but nor for <meta content="2015-12-15T10:10:04.000Z"/>

Why doesn’t it work for meta content="..."