CSS yazıyı ortalama

<!--Main Section-->

        <section id="mainSlider" class="slider">
        <div id="mainCaption" class="caption">
            <h1>Lorem Ipsum is simply dummy text of the printing and typesetting industry</h1>
            <p>Lorem Ipsum has been the industry's</p>
        </div>
    </section>

#mainSlider {
    height: 500px;
    background: url('../img/motormain.jpg') center center no-repeat;
    background-size: cover;
}
.slider {
    width: 100%;
}
#mainCaption {
    width: 500px;
}
.caption {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    width: 100%;
    color: #fff;

}

text-align: center

top: 50%;
left: 50%;

.slider {
    display: flex;
    align-items: center;
    justify-content: center;
}

#mainSlider {
    height: 500px;
    background: url('https://4.bp.blogspot.com/-1X4W9-GXAvA/WU4kV_9x4HI/AAAAAAAABb8/86Pjgp8kBCM0EOoHimIg4QT4i6vSrwi4ACLcBGAs/s1600/chevrolet_camaro_muscle.jpg') center center no-repeat;
    background-size: cover;
    position:relative;
}
.slider {
    width: 100%;
}
#mainCaption {
    width: 500px;
    position:absolute;
    top:40%;
    left:5%;
    z-index:100;
}
.caption {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    width: 100%;
    color: #fff;

}

<section id="mainSlider" class="slider">
        <div id="mainCaption" class="caption">
            <h1>Lorem Ipsum is simply dummy text of the printing and typesetting industry</h1>
            <p>Lorem Ipsum has been the industry's</p>
        </div>
    </section>