JSFiddle - React, Tailwind, and code Playground

HTML

<header>
    <nav>
        <ul>
            <li>Sekce 1</li>
            <li>Sekce 2</li>
            <li>Sekce 3</li>
        </ul>
    </nav>
</header>
<section>
    <div>
        <p id="change">dddd</p>
    </div>
</section>

CSS

nav {
    background-color: black;
    color: white;
    position: fixed;
	top: 0;
	width: 100%;
}

#change {
    background-color: white;
}

JavaScript

$(document).ready(function(){
    $("header").animate({opacity: 0}, 10000);
});