.p-anima-2 {
  position: relative;
}
.p-anima-2::before {
  content: "";
  display: block;
  width: 100%;
  height: 100%;
  background-color: #6667ab;
  position: absolute;
  top: 0;
  left: 0;
  transform: scaleX(0);
  transform-origin: right;
  transition: transform 0.4s;
}
.p-anima-2:hover::before {
  transform: scaleX(1);
  transform-origin: left;
}
.p-anima-2 span {
  position: relative;
  z-index: 1;
}

/* アニメーションとは関係のないスタイル（ボタンの見た目など） */
body {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
}
.c-btn {
  display: block;
  color: #fff;
  font-size: 20px;
  font-weight: bold;
  text-align: center;
  background-color: #333;
  border-radius: 3px;
  width: 300px;
  margin: auto;
  padding: 20px 0;
  text-decoration: none;
}