JSFiddle - React, Tailwind, and code Playground

HTML

<span class="main_title">Заголовок</span>

CSS

.main_title {
    font-size: 16px;    
    line-height: 1.4;
    display: block;
    position: relative;
}

.main_title:after {
    content: '';
    position: absolute;
    width: 100%;
    top: 13px;
    height: 1px;
    display: inline-block;
    border-bottom: 1px solid red;
}