JSFiddle - React, Tailwind, and code Playground

HTML

<p>I don't know where "outer" div 100% width comes from?</p>

<div class="outer">
 <div class="item">
     <p>Something big!</p>
 </div>
</div>

I always thought it'd look like that:

<div class="allright"></div>

CSS

.outer {
    border: solid 1px red;
}

.item {
    height: 300px;
    width: 400px;
    border: solid 1px blue;
}

.allright {
    height: 300px;
    width: 400px;
    border: solid 1px blue;
    outline: solid 1px red;
}