Ok This Thread did helped me a LOT!
So now my function works (i can embeld HTML5 videos) and it looks like this:
function validate_setting($plugin_options){
global $allowedtags;
$allowedtags = array(
'a' => array(
'href' => array (),
'title' => array ()),
'b' => array(
'style'=> array(),
),
);
$allowedtags['video'] = array(
'width' => true,
'height' => true
);
$allowedtags['source'] = array(
'src' => true,
'type' => true
);
$plugin_options['text_area'] = wp_kses($plugin_options['text_area'],$allowedtags);
return $plugin_options;
}