What a Mobile App Needs from Typography
Mobile typography needs three things. First, a tall x-height. At 12px and 13px, the lowercase letters (a, c, e, m, n, o, r, s) are what your eye actually reads. A tall x-height keeps those letters open and readable. Second, open counters. The enclosed white spaces inside letters like a, e, o close up at small sizes on a low-DPI screen. Open counters stay readable. Third, a weight range that covers body (400), labels and buttons (500), and screen headings (600 to 700). Variable fonts are ideal because they cover the full range in one file, which matters on mobile where every kilobyte is paid for in load time.