Fuente: https://kellenmace.com/add-unfiltered_html-capability-to-admins-or-editors-in-wordpress-multisite/

<?php

/**
* Enable unfiltered_html capability for Editors.
*
* @param array $caps The user's capabilities.
* @param string $cap Capability name.
* @param int $user_id The user ID.
* @return array $caps The user's capabilities, with 'unfiltered_html' potentially added.
*/
function km_add_unfiltered_html_capability_to_editors( $caps, $cap, $user_id ) {

if ( 'unfiltered_html' === $cap && user_can( $user_id, 'editor' ) ) {

$caps = array( 'unfiltered_html' );

}

return $caps;
}
add_filter( 'map_meta_cap', 'km_add_unfiltered_html_capability_to_editors', 1, 3 );