code: function percentage(target, context) {
    if (target.slice(-1) === "%" || context.slice(-1) === "%") {
      throw new Error(`Cannot calculate percentage; one or more units appear to
        be %. Units must be rem or px.`);
    }
  
    if (target.slice(-1) !== context.slice(-1)) {
      throw new Error("Cannot calculate percentage; units do not appear to match.");
    }
  
    const unit = target.slice(-1) === "x" ? "px" : "rem";
    const numbers = _getNumbers([target, context], "static", unit);
    const result = numbers.reduce((a, b) => a / b) * 100;
    return _affixValue(result, "fluid");
  }
docs: Calculate a percent value from two static values
  @todo   Add better regex matching to validate rem or px only
  @param  {String} target  The width of the element that will be converted to a percent
  @param  {String} context The width of the element's container
  @return {String}         Converted value
  @usage
  percentage("12rem", "120rem");