Want a Valigned block – NotOK
header class="blockL pw6" style="??min-height:max(300px, max-content)??; div class="centered p1"
.blockL {text-align:left;}
.blockL:before {content:''; display:inline-block; height:100%; vertical-align:middle; margin-right:-0.25em;}
.centered {display:inline-block; vertical-align:middle;}