Under or over
Placing one element below another may sound just like a fraction, but the 'munder', 'mover', and 'munderover' tags are available to do just that.
<munder>
<math><munder><mrow><mn>12</mn><mo>+</mo><mn>24</mn></mrow><mo>⏟</mo></munder></math>
Gives:
<mover>
<math><mover><mrow><mn>12</mn><mo>+</mo><mn>24</mn></mrow><mo>︷</mo></mover></math>
Gives:
<munderover>
<math><munderover><mrow><mn>12</mn><mo>+</mo><mn>24</mn></mrow><mo>⏟</mo><mo>︷</mo></munderover></math>
Gives: