JSFiddle - React, Tailwind, and code Playground

HTML

<div id="main">
    <div id="diff"></div>
    <fieldset>
        <input type="text" size="10" />
        <input type="text" />
    </fieldset>
</div>

CSS

#main {
    width: 100%;
}
input {
    width:100%; /* Expannd to parent (#main). Needed to override default */
    max-width: 100%; /* This will override 'width' to ensure element fits inside parent */
}
#diff {
    height: 1em;
    background-color: red;
}