.child { width: 100%; }

.child { align-self: stretch; }

.parent { align-items: stretch; }

.grandparent {
   float:left;
   clear: both;
   width:100%; //fix for IE11 text overflow
   }

.parent {
  display: flex;
  border: 1px solid red;
  align-items: center;
 }
.child {
  border: 1px solid blue;
 }

max-width: 100%;