Meaning of Font Size and Em Height

What Does Font Size Mean Exactly?

The CSS font size of a font is the “em height”.

“em height” is the height of a design box around the font that includes space for ascender and descender plus a little padding.

The actual height of letter M, differs depending on the font used.

Some character, such as Ü, may extend beyond this design box.

Different font with same size may actually be quite different in perceived size or bounding box size, because there's no standard ratio of x-height of a font to its em height.