JSFiddle - React, Tailwind, and code Playground

HTML

<div class="box">
      <div class="column1">
          a<br>b
      </div>
      <div class="column2">
         a
      </div>
   </div>

CSS

.box {
    width:100px;
    display:box;
    /* Firefox */
    display:-moz-box;
    -moz-box-orient:horizontal;
     /* Safari, Opera, and Chrome */
    display:-webkit-box;
    -webkit-box-orient:horizontal;
    /* W3C */
    display:box;
    box-orient:horizontal;
}

.box .column1 {
    -moz-box-flex:1.0; /* Firefox */
    -webkit-box-flex:1.0; /* Safari and Chrome */
    -ms-flex:1.0; /* Internet Explorer 10 */
    box-flex:1.0;
    background: yellow;
}

.box .column2 {
    -moz-box-flex:1.0; /* Firefox */
    -webkit-box-flex:1.0; /* Safari and Chrome */
    -ms-flex:1.0; /* Internet Explorer 10 */
    box-flex:1.0;
    background: green;
}