Good typography acts like a crystal goblet — it exists solely to present the wine (the content) perfectly, without drawing attention to itself. Here are the cardinal rules for unnoticeable, perfect typography.
The magic numbers: measure and leading
The "measure" is the length of a line of text. For optimal reading comfort, keep your measure between 45 and 75 characters (including spaces). Anything longer causes eye fatigue; anything shorter breaks the reading rhythm.
"Leading" (line height) should generally be 140% to 160% of the font size. A 16px font needs roughly a 24px line height.
Contrast is non-negotiable
Dark grey (not pure black) on a light grey or off-white background reduces eye strain. Ensure your text contrast meets at least the WCAG AA standard (4.5:1 ratio).