JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent">
<div class='content'>
    sdaohdosahdaf
    <br />adfafhaskldjs sgdafadkfjas
    <br />saodfhdpaofa]sdoas [fdf asfasfjasfdkasfad;sdlafa dfasds dad gad
</div>
<div class='sidebar'>
    daobf haohf asod agsdjfa
    <br />sidja ofhad
    <br />sojgh fadpfjas
</div>
</div>

CSS

.parent {
  position: relative;
}

.content {
    background-color: lightblue;
    width: 60%;
    float: left;
}
.sidebar {
    top: 0px;
    right: 0px;
    width: 38%; /* 100% - .content.width */
    float: left;
    background-color: lightgreen;
}
/* Responsive bit*/
@media only screen and (max-width: 320px) {
.content {
    background-color: orange;
    width: 100%;
}
.sidebar {
    background-color: red;
    width: 100%;
}
}