Definition
font-weight exists to control the level of boldness that you can apply to text within HTML elements.
Here are all of the available weights that you have to choose from:
// Shortcuts
font-weight: normal;
font-weight: bold;
// Relative to parent
font-weight: lighter;
font-weight: bolder;
// Numeric values
font-weight: 100;
font-weight: 200;
font-weight: 300;
font-weight: 400;
font-weight: 500;
font-weight: 600;
font-weight: 700;
font-weight: 800;
font-weight: 900;