See documentation for the Mathematical Markup Language (MathML) 2.0 Tag Set.
The @alttext attribute is used only within the mathematical expressions of MathML. The equivalent function, providing words that can be display “behind” a graphic or be pronounced over a graphic, is performed by the <alt-text> element for graphics (and other elements) within the Book Tag Set and the Collection Tag Set.
|Text, numbers, or special characters||See MathML documentation.|
|Restriction: This attribute may be specified if the element is used.|