The font-size-adjust property is used in CSS and certain HTML elements. It specifies what font size should be chosen based on the height of lowercase letters rather than the height of capital letters. This is useful since the legibility of fonts, especially at small sizes, is usually determined by the size of lowercase letters rather than by the size of capital letters. This can cause problems when the first-choice font-family is unavailable and its replacement has a significantly different aspect ratio (the ratio of the size of lowercase letters to the size of the font).