ckeditor iframe not showing custom css code example

Example: ckeditor iframe not showing custom css

Edit the 'contents.css' file that is located in the main CKEditor directory and 
add font-face there for displaying custom font inside the iframe

inside 'ckeeditor/contents.css'

@font-face {
    font-family: 'PRKSI';
    src: url('/fonts/PRKSI.ttf') format('truetype');
}