JSFiddle - React, Tailwind, and code Playground

HTML

<div class="news">
	<h1>Підпишись на новини!</h1>
    <div class="border"></div>
</div>

CSS

.news {
    width: 50%;
    border: 1px solid #000;
    padding: 10px;
    text-align: center;    
}

h1 {
    display: inline;
}

.border {
    height: 1px;
    background-color: #000;
    margin: 0 auto;
}

JavaScript

function resizeHandler() {
    var borders = document.getElementsByClassName('border')
    for (var i = 0; i < borders.length; i++) {
        borders[i].style.width=borders[i].previousElementSibling.offsetWidth + 'px'
    }
}
resizeHandler()
window.onresize = resizeHandler