/* Any Element With Class Title */
            .title {
            }
            
            /* Element with id attribute by value nav */
            #nav {
            }
            
            /* any div element */
            div {
            }
            
            /* any h2 element */
            h2 {
            }
        

            
                
                <link rel="stylesheet" href="css/file.css" />
                
                
                <style>
                p {
                    color: red;
                }
                </style>
                
                
                <p style="color: blue;">This Is Our Paragraph</p>
            
            
        
        

            
            
                <link rel="stylesheet" href="assets/master.css">
                <link rel="stylesheet" href="../source/css/main.css">
            
        

        
            /* valid */
            ._user-name {
            }
            
            /* valid */
            .-user-name {
            }
            
            /* valid */
            .-user-name {
            }
            
            /* Notvalid */
            .1user-name {
            }
            
            /* Notvalid */
            .@user-name {
            }
            
            /* Notvalid */
            .user@name {
            }
            
            /* valid */
            ._user10name {
            }
            
            /* valid */
            .u {
            }
        
        

            /* Bad */
            .USERNAME {
            }
            
            /* Bad */
            .UserName {
            }
            
            /* Good */
            .user-name {
            }
            
            /* Bad */
            .userName {
            }
            
            /* Bad */
            .usernameprofile {
            }