Invalid use of <b> tag

The fieldset of $form['add_redirect'] is wrapped in a <b> tag which is invalid HTML. It also causes all the text within the fieldset to become bold which is needless.

The reason this <b> is invalid is because it's an inline-level element and inline elements are not allowed to contain block-level elements, such as fieldsets and divs.

Drupal core also uses <strong> instead of <b> for semantic reasons.