SOURCE

<section class="layout layout-flex">
    <div class="item">flex</div>
</section>
<section class="layout layout-grid">
    <div class="item">grid</div>
</section>
<section class="layout layout-absolute">
    <div class="item">absolute</div>
</section>
<section class="layout layout-transform">
    <div class="item">transform</div>
</section>
<section class="layout layout-vertical-align-1">
    <div class="item">vertical-align-1</div>
</section>
<section class="layout layout-vertical-align-2">
    <div class="item">vertical-align-2</div>
</section>

.layout {
    width: 200px;
    height: 200px;
    border: 1px solid;
    font-size: 25px;
}
.item {
    width: 100px;
    height: 100px;
    border: 10px solid green;
}

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

.layout-grid {
    display: grid;
    justify-content: center;
    align-content: center;
}

.layout-absolute {
    position: relative;
}
.layout-absolute .item {
    position: absolute;
    left: 0;
    right: 0;
    top: 0;
    bottom: 0;
    margin: auto;
}

.layout-transform .item {
    position: relative;
    top: 50%;
    left: 50%;
    transform: translateX(-50%) translateY(-50%);
}

.layout-vertical-align-1 {
    display: table-cell;
    vertical-align: middle;
    text-align: center;
}
.layout-vertical-align-1 .item {
    display: inline-block;
}

.layout-vertical-align-2 {
    text-align: center;
}
.layout-vertical-align-2::before {
    content: '';
    display: inline-block;
    height: 100%;
    vertical-align: middle;
    width: 0;
}
.layout-vertical-align-2 .item {
    vertical-align: middle;
    display: inline-block;
}