Add HTML Snippet only on specific pages

You can try using the wp_head hook like this and checking if it’s a post or page you want it to load on with an if statement:

function wse_add_snippet_to_header() {
   //replace the array with your own post ids that you want this on
   //replace is_single with is_page if you want to check for pages instead of posts
   if (is_single(array(1234, 1254) {
      ?>
      <html xmlns="http://www.w3.org/1999/xhtml" xmlns:dio="http://docs.dionera.com/namespace/DioML">
      <?php
   }

}
add_action('wp_head', 'wse_add_snippet_to_header');

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