JSFiddle - React, Tailwind, and code Playground

HTML

<div id="A">
    <div id="A1">A1</div>
    <div id="A2">
        <div id="A2-container">
            <div id="A2a">A2a</div>
            <div id="A2b">A2b</div>
        </div>
    </div>
</div>
<div id="B">
    <div id="B1">B1</div>
    <div id="B2">
        <div id="B2-container">
            <div id="B2a">B2a</div>
            <div id="B2b">B2b</div>
        </div>
    </div>
    
</div>

CSS

* {
    margin: 0;
    padding: 0;
    font-family: Arial;        
}

#A {
    position: absolute;
    top: 0px;
    left: 0px;
    background: black;
    width: 50%;
    height: 100%;
    
    display: -moz-box;
    display: -webkit-box;    
    display: box;
    -moz-box-orient: horizontal;
    -webkit-box-orient: horizontal;
    box-orient: horizontal;
}

#A1 {
background: brown;
width: 100px;
height: 80%;   
}

#A2 {
background: orange;
height: 80%;
-moz-box-flex: 1;
-webkit-box-flex: 1;
box-flex: 1;    
}

#A2-container {
    display: -moz-box;
    display: -webkit-box;    
    display: box;
    -moz-box-orient: horizontal;
    -webkit-box-orient: horizontal;
    box-orient: horizontal;
    width: 100%;
    height: 100%;    
}

#A2a {
    background: red;
    height: 80%;   
    -moz-box-flex: 1;
    -webkit-box-flex: 1;
    box-flex: 1;    
}

#A2b {
    background: blue;
    width: 100px;
    height: 80%;
}

#B {
    position: absolute;
    top: 0px;
    right: 0px;
    background: gray;
    width: 50%;
    height: 100%;
    
    display: -moz-box;
    display: -webkit-box;    
    display: box;
    -moz-box-orient: vertical;
    -webkit-box-orient: vertical;
    box-orient: vertical;

}

#B1 {
    background: brown;
    width: 80%;
    height: 100px;   
}

#B2 {
    background: orange;
    width: 80%;
    -moz-box-flex: 1;
    -webkit-box-flex: 1;
    box-flex: 1;    
}

#B2-container {
    display: -moz-box;
    display: -webkit-box;    
    display: box;
    -moz-box-orient: vertical;
    -webkit-box-orient: vertical;
    box-orient: vertical;
    width: 100%;
    height: 100%;    
}

#B2a {
    background: red;
    width: 80%;   
    -moz-box-flex: 1;
    -webkit-box-flex: 1;
    box-flex: 1;    
}

#B2b {
    background: blue;
    width: 80%;
    height: 100px;
}