JSFiddle - React, Tailwind, and code Playground

HTML

<div class="div1">
    <fieldset class="fieldset">
        <legend>test</legend>
        <div class="div2"></div>
    </fieldset>
</div>
<br />
<br />
<div class="div1">
    <fieldset class="fieldset">
        <div class="div2"></div>
    </fieldset>
</div>

CSS

.div1{
    width:400px; 
    height:200px; 
    background-color:teal;
    position:relative;
}
.fieldset{
    position:absolute;
    top:0px;
    right:0px;
    bottom:0px;
    width:50%;
    background-color:red; 
    vertical-align:top;
}
.div2{
    position:absolute; 
    top:15px;
    bottom:0px;
    right:0px;
    left:0px;
    background-color:blue;
}