Unitless line heights appear to have an issue where the size of the element rendered to screen sometimes renders at a different size to the computed line-height. Take the following CSS: html { font-size: 125%; // 20px computed size } p { font-size: 1.5rem; // 30px computed size line-height: 1.33333333333; // 40px computed size } Here's a codepen that demonstrates the issue: http://codepen.io/tbredin/pen/ONNGmY In the inspector this reports the computed line-height as 40px. However the element only takes up 39px of space in the browser. Over long distances this can add up to a big error, is pretty severe (for example; vertical rhythm systems across multiple columns are busted really badly!)