body.small { font-size: 14px; }
body.medium { font-size: 16px; }
body.large { font-size: 20px; }

/* 所有文字都用 em 或 rem 单位，自动继承 body 的字体大小 */
p, h1, h2, h3, span, li {
  font-size: 1em;
  line-height: 1.6;
}


.font-size-selector {
  margin: 20px;
  text-align: center;
}

.font-size-selector span {
  font-family: Arial, sans-serif;
  font-weight: bold;
  margin: 0 10px;
  cursor: pointer;
  user-select: none;
  color: black;
  opacity: 0.6;
  font-size: 14px;
  transition: opacity 0.2s, transform 0.2s;
}

#mediumA {
  font-size: 18px;
}

#largeA {
  font-size: 24px;
}

.font-size-selector span:hover {
  opacity: 1;
  transform: scale(1.2);
}

.font-size-selector span.active {
  color: #007bff;
  opacity: 1;
  text-decoration: underline;
}

.font-size-selector span {
  font-size: 0.9em;
}