Marcin explains why line height works differently in print and the web. Along the way, he hits upon this key insight about CSS:
Web also took away some of the control from typesetters. What in the print era were absolute rules, now became suggestions.
Remember that every line of CSS you write is a suggestion to the browser.
This is a fun game (I scored a measly 73/100). The idea is to develop a feeling for the balance between font-size, line-height, and line length …just like the three sides of an equilateral triangle.
Too many of them still set line-height, font size and line width as independent features when in fact they should all be considered together. The equilateral triangle is a perfect representation of how the three features work in harmony.