Try it Live

<!DOCTYPE html>
<html>
 <head>
 <style>
 .px {
 font-size: 30px ;
 }
 .em {
 font-size: 1.2em;
 }
 .vw {
 font-size: 2.5vw;
 }
 .percent {
 font-size: 70%;
 }
 .small {
 font-size: small;
 }
 .smaller {
 font-size: smaller;
 }
 .x-small {
 font-size: x-small;
 }
 .xx-small {
 font-size: xx-small;
 }
 .medium {
 font-size: medium;
 }
 .large {
 font-size: large;
 }
 .larger {
 font-size: larger;
 }
 .x-large {
 font-size: x-large;
 }
 .xx-large {
 font-size: xx-large;
 }
 </style>
 </head>
 <body>
 Font size in px 
 Font size in em
 Font size in vw(view rt width). Resize your browser window to understand this
 Font size in %
 Font size in small
 Font size in smaller
 Font size in x-small
 Font size in xx-small
 Font size in medium
 Font size in large
 Font size in larger
 Font size in x-large
 Font size in xx-large
 </body>
</html>