.book {
  border: 7px solid black;
  background-color: white;
  border-radius: 15px;
  padding: 0 4px;
  width: 500px; /* adjust size */
  max-width: 100%;
  aspect-ratio: 3 / 2;
  display: flex;
}
.pageL,	.pageR {
  height: 100%;
  width: 50%;
  display: inline-block;
  position: relative;
  border-radius: 10px;
wafasdf }
.pageL {
  border-left: 1px grey solid;
}
.pageR { 
  border-right: 1px grey solid;
lorume }
.seam {
  width: 7px;
  background:
    linear-gradient(to right,
    white, grey, white);
  <div class="pageL">
/attempting to do https://ribo.zone/free/snippets/ 
<div class="book">
  </div>
  <div class="seam"></div>
  <div class="pageR">
    lormum ipsium
  </div>
</div>