JSFiddle - React, Tailwind, and code Playground

HTML

<form id="user_form" name="user_form">
    <input id="firstname" type="text" name="firstname" placeholder="Vorname" required>
    <br>
    <input id="lastname" type="text" name="lastname" placeholder="Nachname" required>
    <br>
    <input id="nickname" type="text" name="nickname" placeholder="Nickname" required>
    <br>
    <input id="email" type="email" name="email" placeholder="Email" required>
    <br>
        <div id="checkbox_div"><input id="conditions" type="checkbox" name="conditions" required>Ich bin mit den <a id="show_2">Teilnahmebedingungen</a> einverstanden!</div>
    <br>
        <div style="clear: both"></div>
    <input id="user_save" type="submit" value="Speichern">
</form>

CSS

#user_form input[type="text"], input[type="email"], input[type="submit"] {
    width: 80%;
    display: block;
    margin: 0 auto;
    height: 50px !important;
    font-size: 18px;
    line-height: normal;
    line-height: 32px\0/;
    /* for IE 8 */
}

#user_form input[type="checkbox"] {
    display: inline !important;
    margin: 0 !important;
    padding: 0 !important;
    height: 50px !important;
    float: left;
}

#checkbox_div {
    width: 80%;
    margin: 0 auto;
    line-height: 50px;
}