Using an image for "0m" (circle mark) or "×" (cross mark) instead of text and providing alternative text for the image which describes the meaning of its mark
Those circle and cross marks are text. Even the Understanding document itself uses them as such. (The "cross mark" is incorrectly chosen. What is depicted is a multiplication symbol.) WAI may wish to look at Unicode characters like U+2611 BALLOT BOX WITH CHECK.
We have reworded the technique title to distinguish between an image of a graphical symbol and a font glyph that happens to have the same appearance but a different meaning.