See documentation for the Mathematical Markup Language (MathML) 2.0 DTD.
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 NCBI Book DTD, the Historical Book DTD, and the NCBI Collection DTD.
| Value | Meaning |
|---|---|
| Data characters | See MathML documentation. |
| Restriction: This attribute may be specified if the element is used. | |