JSFiddle - React, Tailwind, and code Playground

HTML

<fieldset>
    <div>fieldset flexbox</div>
    <div>1</div>
    <div>2</div>
</fieldset>

<div id="parentdiv">
    <div>div flexbox<div>
    <div>3</div>
    <div>4</div>
</div>

CSS

fieldset {
    display: flex;
}

fieldset div {
    display: flex;
}

#parentdiv {
    display: flex;
}

#parentdiv div {
    display: flex;
}