1
for the chess effect, you should do as follows.:
.container {
  width: 560px;
}
.container div {
  float: left;
  width: 50%;
  height: 40px;
}
.container div:nth-child(4n - 3), 
.container div:nth-child(4n) {
  background-color: gainsboro;
}
.container div:nth-child(4n - 2), 
.container div:nth-child(4n - 1) {
  background-color: silver;
}
<div class="container">
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
</div>



divides them into class 1 (group of lighter colored Ivs), class 2 (group of darker colored Ivs) and to define their positions you put different ids and define the positions individually as well as understand
– Sarah