CodePen

HTML

            
                         <div id="header">
                <h1>Single Page App</h1>
            </div>

            
          
!

↑ Insert the most common viewport meta tag

CSS

            
              
            
          
!
? ?
? ?
Must be a valid URL.
+ add another resource
via CSS Lint

JS

            
              
            
          
!
Must be a valid URL.
+ add another resource
via JS Hint
Loading ..................