CSS grids: align square cells with container edges with consistent grid gap?

Question

The goal is to align square cells with their container\'s leading and trailing edges while achieving a consistent gap between cells on each row, and between each row.

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Answer 1

This one is the answer to your first problem no need to give width to li.

ul {
  display: grid;
  width: 260px;
  grid-template-columns: repeat(auto-fit, minmax(40px, 1fr));
  grid-auto-rows: 1fr;
  grid-gap: 10px;
  list-style-type: none;
  border: 2px solid black;
  box-sizing: border-box;
  margin: 0;
  padding: 5px;
  background: gray;
}

li {
/*   width: 40px; */
  height: 40px;
}