JSFiddle - React, Tailwind, and code Playground

HTML

<form>
    <fieldset>
        <legend>A form</legend>
        <label for="text">A label</label>
        <input id="text" placeholder="A text input" />
    </fieldset>
    <fieldset>
        <div id="container">
        <button>Button A</button>
        <button>Button B</button>
        </div>
    </fieldset>
</form>

CSS

form>fieldset:not(:last-of-type) {
    display:flex;
    flex-direction: column;
}
form>fieldset:not(:last-of-type)>input {
    width: 100%;
}
form>fieldset:last-of-type {
    display: flex;
    flex-direction: row;
}

#container {
   width: 30%;
    margin: 0 auto;
    
}