code: function consumeArgument(stream) {
  	const result = new CSSValue();
  	let value;
  
  	while (!stream.eof()) {
  		stream.eatWhile(isWhiteSpace);
  		value = consumeNumericValue(stream) || consumeColor(stream)
  			|| consumeQuoted(stream) || consumeKeywordOrFunction(stream);
  
  		if (!value) {
  			break;
  		}
  
  		result.add(value);
  	}
  
  	return result.size ? result : null;
  }
docs: Consumes a single argument. An argument is a `CSSValue`, e.g. it could be
  a space-separated string of value
  @param  {StreamReader} stream
  @return {CSSValue}