JSFiddle - React, Tailwind, and code Playground

HTML

<div id="outer">
    <div id="container">
        <div id="fixed">
            Sitename
        </div>               
    </div>
</div>

CSS

#outer{
    width:300px;
    border: 1px solid yellow;   
}

#container {
    width: 90%; /*When I use e.g 250 px it works. But I need it relativ*/
    border: 1px solid red;
    height: 300px;
}

#fixed {
    position: relative;
    width: 100%;
    border: 1px solid green;
}