Wikimedia Bugzilla is closed!

Vector first reverses the order of horizontal nav elements in HTML and then prevents flipping of float:left rules in CSS to make the interface looks the same in LTR and RTL environments.

Similar craziness is done to the search form - the input and the button are rendered in different order in LTR and RTL.

This causes e.g. bug 34587.

Vector.php:
    // Reverse horizontally rendered navigation elements
    if ( $this->data['rtl'] ) {
      $this->data['view_urls'] =
        array_reverse( $this->data['view_urls'] );
      $this->data['namespace_urls'] =
        array_reverse( $this->data['namespace_urls'] );
      $this->data['personal_urls'] =
        array_reverse( $this->data['personal_urls'] );
    }
    ...
    // If there's a series of elements, reverse them when in RTL mode
    } elseif ( $this->data['rtl'] ) {
      $elements = array_reverse( $elements );
    }
    ... etc

vector/screen.css:
    /* @noflip */
    div.vectorTabs {
      float: left;
      height: 2.5em;
    }
    ...
    /* @noflip */
    div.vectorTabs ul {
      float: left;
    }
    ... etc

Why would anyone do that is just mind-boggling to me. I assume it's a workaround for some IE6 bugs.