SOURCE

<div class="root">
    <div class="parent">
       <div class="child">1</div>
    </div>

   <div class="child">2</div>
</div>

.root {
    display: flex;
    flex-direction: row;
    justify-content: flex-start;
}

.parent div{
    
        width: 20px;
        height: 20px;
        border: red 1px solid;
        background-color: #ffff00;
} 

.parent {
    div {
        width: 20px;
        height: 20px;
        border: red 1px solid;
        background-color: #ffff00;
    }
} 

.child {
    width:20px;
    height: 20px;
    border: red 1px solid;
    background-color: green;
}