Inline mathematical formulas are normally vertically aligned with respect to the so-called "math axis" (e.g. vertical position of the center of minus sign or fraction bar). In the old PNG mode, the images are aligned with the CSS rule
vertical-align: center, which is good enough in some cases but is incorrect in others. The math axis is now used in the MathML mode with SVG fallback. For example or should be aligned with respect to the math axis.