通过 CSS 让 html 网页中的内容不可选

* {
  moz-user-select: -moz-none;
  -moz-user-select: none;
  -o-user-select: none;
  -khtml-user-select: none;
  -webkit-user-select: none;
  -ms-user-select: none;
  user-select: none;
}