header {
  background-color: lightgray;
  position: sticky;
  top: 0;
  align-items: center;
}

header > ul > li {
  display: inline-block;
}

#page-title {
  color: red;
}

.container {
  width: 100%;
  height: 200px;
  background-color: lightpink;
}

.flex-container {
  display: flex;
  align-items: center;
  justify-content: space-around;
}

.grid-container {
  display: grid;
  grid-template-columns: 1fr 1fr 1fr;
  grid-template-rows: 1fr 1fr 1fr;
}

@media screen and (max-width: 700px) {
  .grid-container {
    grid-template-columns: 1fr;
  }
}

.child-base {
  width: 50px;
  height: 50px;
  background-color: lightblue;
  font-size: 10px;
  border: 1px solid black;
}

.c1 {
  position: absolute;
  top: 200px;
  left: 200px;
}

.c2 {
  position: relative;
  top: 10px;
  left: 10px;
}

.c3 {
  position: fixed;
  top: 200px;
  left: 100px;
}

.section1 {
  color: green;
}

.section2 div {
  background-color: yellow;
}

.section3 > div::selection {
  background-color: yellow;
}

div {
  background-color: lightcyan;
}

h1 {
  color: blue;
}

h2::first-letter {
  background-color: orange;
  color: white;
}

.section1 > p::before {
  content: "!あ";
  color: red;
  background-color: yellow;
}

.section1 > p:hover::after {
  content: "あ!";
  color: red;
  background-color: yellow;
}

.section1 > p:nth-child(even) {
  background-color: blue;
  color: white;
}

.section1 > p:hover {
  background-color: green;
  color: white;
}