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;
}

<ul class="palette">
  <li style="background-color: rgb(0, 0, 255);"></li>
  <li style="background-color: rgb(51, 51, 51);"></li>
  <li style="background-color: rgb(203, 58, 135);"></li>
  <li style="background-color: rgb(237, 106, 90);"></li>
  <li style="background-color: rgb(155, 193, 188);"></li>
  <li style="background-color: rgb(255, 247, 174);"></li>
  <li style="background-color: rgb(184, 51, 106);"></li>
  <li style="background-color: rgb(61, 44, 46);"></li>
  <li style="background-color: rgb(105, 173, 212);"></li>
  <li style="background-color: rgb(245, 223, 22);"></li>
  <li style="background-color: rgb(252, 186, 86);"></li>
  <li style="background-color: rgb(0, 185, 252);"></li>
  <li style="background-color: rgb(181, 141, 182);"></li>
  <li style="background-color: rgb(58, 50, 96);"></li>
</ul>

Answer 2

The issue is that the grid cells are fine but the content inside (li) is not matching them.

Instead of using fixed width/height on the li you can consider percentage value and they will be a bit bigger in some cases but will remain square elements:

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: 0;
  background: gray;
  animation:change 5s linear alternate infinite;
}

li {
  width: 100%;
  padding-top:100%;
}

@keyframes change {
  to {width:120px}
}

<ul class="palette">
  <li style="background-color: rgb(0, 0, 255);"></li>
  <li style="background-color: rgb(51, 51, 51);"></li>
  <li style="background-color: rgb(203, 58, 135);"></li>
  <li style="background-color: rgb(237, 106, 90);"></li>
  <li style="background-color: rgb(155, 193, 188);"></li>
  <li style="background-color: rgb(255, 247, 174);"></li>
  <li style="background-color: rgb(184, 51, 106);"></li>
  <li style="background-color: rgb(61, 44, 46);"></li>
  <li style="background-color: rgb(105, 173, 212);"></li>
  <li style="background-color: rgb(245, 223, 22);"></li>
  <li style="background-color: rgb(252, 186, 86);"></li>
  <li style="background-color: rgb(0, 185, 252);"></li>
  <li style="background-color: rgb(181, 141, 182);"></li>
  <li style="background-color: rgb(58, 50, 96);"></li>
</ul>