code: function getDurationClass ( duration ) {
  
      const cls = `slide-${duration}`,
            selector = `.${cls}`;
  
      if ( !EmbeddedCSS.get ( selector ) ) {
  
        const seconds = duration / 1000;
  
        EmbeddedCSS.set ( selector, 'transition', `height ${seconds}s, width ${seconds}s, padding ${seconds}s, border-width ${seconds}s !important` );
  
      }
  
      return cls;
  
    }
docs: /* UTILITIES