JSFiddle - React, Tailwind, and code Playground

HTML

<header>
    <h1>THE CODE REVIEW</h1>
    <h2>Signup for our newsletter</h2>
    <p>Get the latest news on how your code is doing right in your inbox</p>
</header>

CSS

header {
    position: relative;
    padding-bottom: 2em;
    /* border-bottom-style: solid; */
    /* border-bottom-color: #339966; */
    /* border-bottom-width: 2px; */
}

header::before {
    content: "";
    position: absolute;
    z-index: -1;
    top: 0;
    /* left: 0; */
    /* right: 0; */
    bottom: 6px;
    border-bottom: 5px solid #339966;
    width: 80vw;
    left: 50%;
    transform: translateX(-50%);
}

header::after {
    content: "";
    position: absolute;
    z-index: -1;
    top: 0;
    /* left: 0; */
    /* right: 0; */
    bottom: 0;
    border-bottom: 2px solid #339966;
    width: 80vw;
    left: 50%;
    transform: translateX(-50%);
}