JSFiddle - React, Tailwind, and code Playground

HTML

<form action="" method="get">
    <fieldset>
        <legend>Elije el mes y día de tu nacimiento:</legend>
        <label for="mes">Mes:</label>
        <select id="mes" name="mes">
            <option value="">-- selecciona un mes --</option>
        </select>
        <label for="dia">Día:</label>
        <select id="dia" name="dia">
            <option value="">-- selecciona un día --</option>
        </select>
    </fieldset>

    <fieldset>
        <legend>Resultados:</legend>
        <label for="inputSigno">Horóscopo:</label>
        <input type="text" id="inputSigno" name="inputSigno">

        <p>Tu signo para el horóscopo es: <span id="textSigno"></span></p>
    </fieldset>
</form>

JavaScript

function generateDays() {
            var days = "";
            for (var i = 1; i <= 31; i++) {
                days += '<option value="' + i + '">' + i + '</option>';
            }
            return days;
        }
        function generateMonths() {
            var months = "";
            for (var i = 1; i <= 12; i++) {
                months += '<option value="' + i + '">' + i + '</option>';
            }
            return months;
        }
        function generateSign(mes, dia) {
            var zodiaco = "";
            switch (mes) {
                case "1": // 01
                    if (dia >= 21)
                        zodiaco = "acuario";
                    break;
                case "2": // 02
                    if (dia <= 19)
                        zodiaco = "acuario";
                    else
                        zodiaco = "piscis";
                    break;
                case "3": // 03
                    if (dia <= 20)
                        zodiaco = "piscis";
                    else
                        zodiaco = "aries";
                    break;
                case "4": // 04
                    if (dia <= 20)
                        zodiaco = "aries";
                    else
                        zodiaco = "abril";
                    break;
                case "5": // 05
                    if (dia <= 21)
                        zodiaco = "tauro";
                    else
                        zodiaco = "geminis";
                    break;
                case "6": // 06
                    if (dia <= 21)
                        zodiaco = "geminis";
                    else
                        zodiaco = "cancer";
                    break;
                case "7": // 07
                    if (dia >= 23)
                        zodiaco = "cancer";
                    else
                        zodiaco = "leo";
                    break;
                case "8": // 08
                    if (dia <= 23)
  ...