<div class="grid-lines"></div>

.grid-lines {
  --n: 3; /* number of rows */
  --m: 5; /* number of columns */
  --s: 80px;  /* control the size of the grid */
  --t: 2px;   /* the thickness */

  width: calc(var(--m)*var(--s) + var(--t));
  height: calc(var(--n)*var(--s) + var(--t));
  background:  
    conic-gradient(from 90deg at var(--t) var(--t),#0000 25%,#000 0)
     0 0/var(--s) var(--s);
}
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