how to change font size in html

HTML

<body>      
         <h1 style="color:red;font-size:40px;">Heading</h1>      
         <p style="color:blue;font-size:18px;">This is demo text</p>    
      </body><span style="font-size:20px;"></span>font-size: 25px;font-size:20px;

"default size is : +-16px;"  <font size="+2">This is bigger text.</font>

Source