extended_valid_elements
не лучший выбор здесь.
Посмотрите на настройку valid_elements
. Вот пример
// The valid_elements option defines which elements will remain in the edited text when the editor saves.
valid_elements: "@[id|class|title|style|data-options|data*]," +
"a[name|href|target|title]," +
"#p,-ol,-ul,-li,br,img[src|height|width],-b,-i,-u," +
"-span[data-mce-type],hr",