code: def remove_unnecessary_whitespace(css):
      
  
      def pseudoclasscolon(css):
  
          """
          Prevents 'p :link' from becoming 'p:link'.
  
          Translates 'p :link' into 'p ___PSEUDOCLASSCOLON___link'; this is
          translated back again later.
          """
  
          regex = re.compile(r"(^|\})(([^\{\:])+\:)+([^\{]*\{)")
          match = regex.search(css)
          while match:
              css = ''.join([
                  css[:match.start()],
                  match.group().replace(":", "___PSEUDOCLASSCOLON___"),
                  css[match.end():]])
              match = regex.search(css)
          return css
  
      css = pseudoclasscolon(css)
      # Remove spaces from before things.
      css = re.sub(r"\s+([!{};:>+\(\)\],])", r"\1", css)
  
      # If there is a `@charset`, then only allow one, and move to the beginning.
      css = re.sub(r"^(.*)(@charset \"[^\"]*\";)", r"\2\1", css)
      css = re.sub(r"^(\s*@charset [^;]+;\s*)+", r"\1", css)
  
      # Put the space back in for a few cases, such as `@media screen` and
      # `(-webkit-min-device-pixel-ratio:0)`.
      css = re.sub(r"\band\(", "and (", css)
  
      # Put the colons back.
      css = css.replace('___PSEUDOCLASSCOLON___', ':')
  
      # Remove spaces from after things.
      css = re.sub(r"([!{}:;>+\(\[,])\s+", r"\1", css)
  
      return css
docs: Remove unnecessary whitespace characters.