JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <input type="text" />
    <div class="filler"></div>
</div>

CSS

div.container {
    padding: 5px;
    border: 1px solid black;
    background-color: gray;
}

div.filler {
    background-color: red;
    height: 5px;
}

input {
    display: block;
    width: 100%;
    box-sizing: border-box; /* css3 rec */
    -moz-box-sizing: border-box; /* ff2 */
    -ms-box-sizing: border-box; /* ie8 */
    -webkit-box-sizing: border-box; /* safari3 */
    -khtml-box-sizing: border-box; /* konqueror */
}