h2 {
  text-align: center;
}

h2::before {
  margin: auto;
  content: "początek ";
  color: red;
}
h2::after {
  margin: auto;
  content: " koniec";
  color: blue;
}
.d {
  height: 100px;
  width: 100px;
  background-color: red;
  margin: auto;
}
h2::selection {
  color: cyan;
}
.d:hover {
  background-color: black;
}
.a::before {
  width: 20px;
  height: 20px;
  display: inline-block;
  content: "";
  background-color: red;
}
.a::after {
  width: 20px;
  height: 20px;
  display: inline-block;
  content: "";
  background-color: blue;
}
.b:nth-child(5n) {
  margin: auto;
  width: 500px;
  height: 30px;
  background-color: red;
}
.b:nth-child(5n + 1) {
  margin: auto;
  width: 500px;
  height: 30px;
  background-color: orange;
}
.b:nth-child(5n + 2) {
  margin: auto;
  width: 500px;
  height: 30px;
  background-color: yellow;
}
.b:nth-child(5n + 3) {
  margin: auto;
  width: 500px;
  height: 30px;
  background-color: green;
}
.b:nth-child(5n + 4) {
  margin: auto;
  width: 500px;
  height: 30px;
  background-color: blue;
}
.b:hover {
  background-color: white;
}
.c {
  text-align: center;
  color: blueviolet;
}