CodePen

HTML

            
              <picture>
   <source srcset="http://placehold.it/500x300" media="(min-width: 40em)">
   <source srcset="http://placehold.it/250x150">
   <img src="http://placehold.it/250x150" alt="A placeholder image">
</picture>
            
          
!

↑ 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 ..................