JSFiddle - React, Tailwind, and code Playground

HTML

<div class="row-flex">
  <div> hello 1</div>
  <div> hello 2</div>
</div>

CSS

.row-flex {
    --flex-basis: 30%;
    display: flex;
}

.row-flex >:first-child {
     flex: 0 0 var(--flex-basis);
}