docs: function keyframes(content, preservedTokens) {
  
      const pattern = /@[a-z0-9-_]*keyframes\s+[a-z0-9-_]+\s*{/gi
  
      let index = 0
      let buffer
  
      const preserve = (part, i) => {
          part = part.replace(/(^\s|\s$)/g, '')
          if (part.charAt(0) === '0') {
              preservedTokens.push(part)
              buffer[i] = ___PRESERVED_TOKEN_ + (preservedTokens.length - 1) + '___'
          }
      }
  
      while (true) {
  
          let level = 0
          buffer = ''
  
          let startIndex = content.slice(index).search(pattern)
          if (startIndex < 0) {
              break
          }
  
          index += startIndex
          startIndex = index
  
          let len = content.length
          let buffers = []
  
          for (; index < len; ++index) {
  
              let ch = content.charAt(index)
  
              if (ch === '{') {
  
                  if (level === 0) {
                      buffers.push(buffer.replace(/(^\s|\s$)/g, ''))
  
                  } else if (level === 1) {
  
                      buffer = buffer.split(',')
  
                      buffer.forEach(preserve)
  
                      buffers.push(buffer.join(',').replace(/(^\s|\s$)/g, ''))
                  }
  
                  buffer = ''
                  level += 1
  
              } else if (ch === '}') {
  
                  if (level === 2) {
                      buffers.push('{' + buffer.replace(/(^\s|\s$)/g, '') + '}')
                      buffer = ''
  
                  } else if (level === 1) {
                      content = content.slice(0, startIndex) +
                          buffers.shift() + '{' +
                          buffers.join('') +
                          content.slice(index)
                      break
                  }
  
                  level -= 1
              }
  
              if (level < 0) {
                  break
  
              } else if (ch !== '{' && ch !== '}') {
                  buffer += ch
              }
          }
      }
  
      return content
  }