• HTML
• CSS

HTML

<h1>Ventana Modal</h1>
<a href="#modal" id="show-modal">Abrir Ventana</a>

<aside id="modal" class="modal">
    <div class="content-modal">
        <header>
            <a href="#" class="close-modal">X</a>
            <h2>Buen Trabajo!</h2>
        </header>
        <article></article>
    </div>
    <a href="#" class="btn-close-modal"></a>
</aside>

CSS

body{
    background-color: #3e2525;
    font-family: sans-serif;
    width: 100%;
    height: 100%;
    margin: 0;
    padding: 0;
    padding-top: 10em;
    position: relative;
    text-align: center;
}

h1 {
    color: white;
    padding: 1em;
}

#show-modal{
    text-align: center;
    background: #bc651a;
    padding: 15px 15px;
    color: white;
    font-weight: 100;
    font-size: 1.5em;
    border-radius: 3.5px;
    text-decoration: none;
    transition: all 0.15s linear;
}

#show-modal:hover{
    cursor: pointer;
    background: #bc9d1a;
}

.modal{
    position: fixed;
    top: -100vh;
    left: 0;
    z-index: 99999999;
    background: rgba(0, 0, 0, 0.75);
    width: 100vw;
    height: 100vh;
    opacity: 0;
    transition: opacity 0.35s ease;
}

.modal .content-modal{
    width: 100%;
    max-width: 500px;
    position: fixed;
    left: 50%;
    top: -100vh;
    transition: top 0.35s ease;
    margin-left: -250px;
    background: white;
    box-shadow: 0 1px 2px rgba(0,0,0,0.1);
    border-radius: 2px;
    z-index: 9999999;
}

.modal h2{
    padding: 0.5em;
    text-align: center;
    color: #444444;
    margin: 0;
}

.modal article{
    height: 300px;
    background: #bc651a;
}

.close-modal{
    color: #e74c3c;
    position: absolute;
    top: 0.2em;
    right: 0.375em;
    margin: 0;
    padding: 5px;
    font-weight: bold;
    font-size: 1.5em;
    text-decoration: none;
}

.modal a:hover{
    color: #c0392b;
}

.modal:target{
    opacity: 1;
    top: 0;
}

.modal .btn-close-modal{
    position: absolute;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: 99999991;
}

.modal:target .content-modal{
    top: 50px;
    transition: top 0.35s ease;
}

@media (max-width: 500px){
    .modal .content-modal{
        width: 90%;
        max-width: none;
        left: 5%;
        margin-left: 0;
    }
    
}