JSFiddle - React, Tailwind, and code Playground

HTML

<fieldset class="login selected">
        <legend class="login_link">Bejelentkezés</legend>
        <div class="form_container">
            <form id="loginform">
                [loginform]dfgsdfgsdfg
              
            </form>
        </div>
    </fieldset>
    <fieldset class="reg">
        <legend class="registration_link">Regisztráció</legend>
        <div class="form_container clearfix">
            <form id="regform">
                [regform]
           </form>
        </div>
    </fieldset>

CSS

.login .form_container {
        display: block;
    }
    .reg .form_container {
        display: none;
    }
    fieldset {
        border: none;
    }
    .selected {
        border: 1px inset #000;
    }
    legend {
        background: #ccc;
        cursor: pointer;
    }

JavaScript

$(".login_link,.registration_link").live("click",function(){
        $(this).parent().parent().children().children('.form_container').toggle();
        $(this).parent().parent().children().toggleClass("selected");
    });