To write the Not A Superset Of symbol ⊅ on keyboard using ALT codes; ALT+8837, just hold down the ALT key while typing the alt key code 8837. You have to use the numeric keypad of your keyboard. If you don not have numeric keypad, hold down the Fn and ALT keys while typing the alt code number.
Download the Not A Superset Of symbol as PNG images at different sizes so you can use in your website or art project.
