<div class="loader"></div>

.loader {
  --s: 70px;
  --g: 15px;  
  
  width: calc(3*(1.353*var(--s) + var(--g)));
  display: grid;
  justify-items: end;
  aspect-ratio: 3;
  outline:1px solid red;
}
.loader:before { 
  content: "";
  width: calc(4*100%/3);
  background:
  conic-gradient(from -90deg at var(--s) calc(0.353*var(--s)),
      #fff 135deg,#666 0 270deg,#aaa 0); 
  --_m:
    linear-gradient(to bottom right,
       #0000 calc(0.25*var(--s)),#000 0 calc(100% - calc(0.25*var(--s)) - 1.414*var(--g)),#0000 0),
    conic-gradient(from -90deg at right var(--g) bottom var(--g),#000 90deg,#0000 0);
   -webkit-mask: var(--_m);
           mask: var(--_m);
  background-size:   calc(100%/4) 100%;
  -webkit-mask-size: calc(100%/4) 100%;
          mask-size: calc(100%/4) 100%;
  -webkit-mask-composite: source-in;
          mask-composite: intersect;
}

body {
  margin: 0;
  background: #000;
  min-height: 100vh;
  display: grid;
  place-content: center;
}