JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html lang="en">
    <head>
        <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=0" />

        <meta charset="utf-8" />
        <title>Responsive menu | Script Tutorials</title>

        <link href="css/main.css" rel="stylesheet" type="text/css" />
    </head>
    <body>

        <!-- menu pocetak -->

        <div class="container">
            <ul id="nav">
                <li><a href="index.html">Početna</a></li>
                <li><a href="#s1">Matematiči kutak</a>
                    <span id="s1"></span>
                    <ul class="subs">
                        <li><a href="#">Header a</a>
                            <ul>
                                <li><a href="#">Submenu x</a></li>
                                <li><a href="#">Submenu y</a></li>
                                <li><a href="#">Submenu z</a></li>
                            </ul>
                        </li>
                        <li><a href="#">Header b</a>
                            <ul>
                                <li><a href="#">Submenu x</a></li>
                                <li><a href="#">Submenu y</a></li>
                                <li><a href="#">Submenu z</a></li>
                            </ul>
                        </li>
                    </ul>
                </li>
                <li class="active"><a href="#s2">Školski matematički dio</a>
                    <span id="s2"></span>
                    <ul class="subs">
                        <li><a href="#">Header c</a>
                            <ul>
                                <li><a href="citanje-i-pisanje.htm">Učimo čitati i pisati</a></li>
                                <li><a href="Mala-web-skola/index.html">Mala web škola</a></li>
                            </ul>
                        </li>
                        <li><a href="#">Header d</a>
                            <ul>
                             ...

CSS

* {
    margin: 0;
    padding: 0;
}
html {
    background-color: #fff;
    height: 100%;
}
body {
    color: #333333;
    font: 0.75em/1.5em Arial,sans-serif;
}
.container {
    margin: 0 auto;
    width: 96%;
}

/* common and top level styles */
#nav span {
    display: none;
}
#nav, #nav ul {
    list-style: none outside none;
    margin: 0;
    padding: 0;
}
#nav {
    background-color: #F5F5F5;
    border-bottom: 5px solid #333333;
    float: left;
    margin-left: 1%;
    margin-right: 1%;
    position: relative;
    width: 98%;
}
#nav ul.subs {
    background-color: #FFFFFF;
    box-shadow: 1px 1px 5px rgba(0, 0, 0, 0.2);
    color: #333333;
    display: none;
    left: 0;
    padding: 2%;
    position: absolute;
    top: 54px;
    width: 96%;
}
#nav > li {
    border-bottom: 5px solid transparent;
    float: left;
    margin-bottom: -5px;
    text-align: left;
    -moz-transition: all 300ms ease-in-out 0s;
    -ms-transition: all 300ms ease-in-out 0s;
    -o-transition: all 300ms ease-in-out 0s;
    -webkit-transition: all 300ms ease-in-out 0s;
    transition: all 300ms ease-in-out 0s;
}
#nav li a {
    display: block;
    text-decoration: none;
    -moz-transition: color 450ms ease-in-out 0s, background-color 450ms ease-in-out 0s;
    -ms-transition: color 450ms ease-in-out 0s, background-color 450ms ease-in-out 0s;
    -o-transition: color 450ms ease-in-out 0s, background-color 450ms ease-in-out 0s;
    -webkit-transition: color 450ms ease-in-out 0s, background-color 450ms ease-in-out 0s;
    transition: color 450ms ease-in-out 0s, background-color 450ms ease-in-out 0s;
    white-space: normal;
}
#nav > li > a {
    color: #333333;
    display: block;
    font-size: 1.3em;
    line-height: 49px;
    padding: 0 15px;
    text-transform: uppercase;
}
#nav > li:hover > a, #nav > a:hover {
    background-color: #F55856;
    color: #FFFFFF;
}
#nav li.active > a {
    background-color: #333333;
    color: #FFFFFF;
}

/* submenu */
#nav li:hover ul.subs {
   ...

JavaScript

$( '#izbor' ).on( 'change', function() {
    
    var vrijednost = $( this ).val();

    $( '#prvi, #drugi' ).hide();
    
    $( 'body' ).find( vrijednost ).show();
    
});