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A high x-height (relative to cap height) makes text appear larger and more legible at small sizes, which is why typefaces designed for screens — such as Georgia and Verdana — have notably tall x-heights. X-height typically ranges from 40% to 75% of the cap height. In CSS, the 'ex' unit is defined as the x-height of the current font, though in practice most browsers approximate it as 0.5 em.