MathML attribute reference - MathML
align <mtable> <munder>, <mover>, <munderover> <mstack> specifies different alignments of several elements (see element pages for details).
... unimplemented charalign <mstack> specifies the horizontal alignment of digits.
... unimplemented decimalpoint <mstyle> if the decimalpoint value is used to specify the alignment, this attribute is specifying the character for the alignment point within <mstack> and <mtable> columns.
... unimplemented stackalign <mstack> stretchy <mo> specifies whether the operator stretches to the size of the adjacent element.

<mstyle> - MathML
decimalpoint this attribute is specifying the character for the alignment point within <mstack> and <mtable> columns, if the decimalpoint value is used to specify the alignment.
... align does not apply to <mtable> or <mstack>.

MathML element reference - MathML
ces) n <mn> (number) o <mo> (operator) <mover> (overscript) p <mpadded> (space around content) <mphantom> (invisible content with reserved space) r <mroot> (radical with specified index) <mrow> (grouped sub-expressions) s <ms> (string literal) <mscarries> (annotations such as carries) <mscarry> (single carry, child element of <mscarries>) <msgroup> (grouped rows of <mstack> and <mlongdiv> elements) <msline> (horizontal lines inside <mstack> elements) <mspace> (space) <msqrt> (square root without an index) <msrow> (rows in <mstack> elements) <mstack> (stacked alignment) <mstyle> (style change) <msub> (subscript) <msup> (superscript) <msubsup> (subscript-superscript pair) t <mtable> (table or matrix) <mtd> (cell in a table or a matrix) <mtext> (text) ...
...close> <merror> <mfenced> <mfrac> <mpadded> <mphantom> <mroot> <mrow> <msqrt> <mstyle> script and limit elements <mmultiscripts> <mover> <mprescripts> <msub> <msubsup> <msup> <munder> <munderover> <none> tabular math <maligngroup> <malignmark> <mlabeledtr> <mtable> <mtd> <mtr> elementary math <mlongdiv> <mscarries> <mscarry> <msgroup> <msline> <msrow> <mstack> uncategorized elements <maction> semantic annotations <annotation> <annotation-xml> <semantics> ...

Mozilla MathML Status
elementary math element [- attribute] notes mstack not implemented.