fontsize measuring type in pixels font-height em-square grid 10px

Move the grid by dragging the right bottom corner of the browserframe. 
This will help you measure font-size and line-height. Counting the pixels by the 10. Font-height=1½ capital-height!
 

Typesetting from the past was not measured with pixels but Didotpoints or Pica-ems. Keep in mind font-height is 1½ capital-height!
‘Em’ equals the width of a classic(!) capital M. The Em-square of a classic type equals the font/corps-height. Or rather it is a lead square as shown in this movie.
The large type above equals em 60px, baseline-height 60px. Each ‘Em’-size in css relates to the one body font-size.