Percentages are a relative unit of measurement similar to EMs
When you size elements based on percentages the resulting size will be relative to that element’s parent
The default font-size in percentages is 100%
Can also be used instead of EMs
Example: Percentage in action
In the example above you will see that the % measurements for the .red-box and .green-box elements are relative to the .big-box element (i.e. their parent)