JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrapper">
    <div class="aside-content">Seitenspalte<br/>Werbung<br/>Infos</div>
    <div class="main-content">Hauptinhalt</div>
    <br class="clear"/><!-- hilft dem Elternelement -->
</div>
<br/><br/><br/>
<div class="wrapper">
    <div class="aside-content">Seitenspalte<br/>Werbung<br/>Infos</div>
    <div class="main-content">Hauptinhalt</div>
</div>

CSS

.wrapper {
    background-color: red;
    width: 800px
}

.aside-content {
    background-color: #3498db;
    float: left;
    width: 200px;
}
.main-content {
    background-color: #bdc3c7;
    overflow: hidden;
}

/* hilft dem Elternelement */
.clear { clear: left; }