Cross Browser Solution -
Test Suites -
3.6.1 MathML - Stacks of Characters mstack

Mstack is used to lay out rows of numbers that are aligned on each digit. The children of an mstack represent rows, or groups of them, to be stacked each below the previous row; there can be any number of rows. An msrow represents a row; an msgroup groups a set of rows together so that their horizontal alignment can be adjusted together; an mscarries represents a set of carries to be applied to the following row; an msline represents a line separating rows. Any other element is treated as if implicitly surrounded by msrow. MathML Fundamentals

<math > <mstack style="border:1px"> <mn style="border:1px">723</mn> <mn style="border:1px" mathsize="70">123</mn> <msrow style="border:5px"> <mn>456</mn> <mo>+</mo> </msrow> <msline style="border:1px"/> <mn style="border:1px">1302</mn> </mstack> </math>