CSS font-size is widely used in web development, in this tutorial, we will use an example to help you understand and use it correctly.
Step 1: create a html page and input this code below in body element
<h4>Title 1: font size is 12px</h4>
Then save it and open it by your browser, you may get the effect like this
Title 1: font size is 12px
Step 2: add css style in this html page
<style type="text/css"> body{font-size:12px;} h4{font-size:100%;} </style>
Then save it, open it again, you may find the size of text in h4 is 12px.
So font-size:100% means this element(h4) will inherit the font size of its parent element(body).