Simple Watermark with CSS Only

html:after {

  /* common custom values */
  content: "© Water Mark"; /* your site name */
  font-size: 720%;         /* font size */
  color: rgba(0, 0, 0, .1);
  /* alpha, could be even rgba(0,0,0,.05) */

  /* rest of the logic */
  z-index: 9999;
  cursor: default;
  display: block;
  position: fixed;
  top: 33%;
  right: 0;
  bottom: 0;
  left: 15%;
  font-family: sans-serif;
  font-weight: bold;
  font-style: italic;
  text-align: center;
  line-height: 100%;

  /* not sure about who implemented what ..
    ... so bring it all */
  -webkit-pointer-events: none;
  -moz-pointer-events: none;
  -ms-pointer-events: none;
  -o-pointer-events: none;
  pointer-events: none;

  -webkit-transform: rotate(-45deg);
  -moz-transform: rotate(-45deg);
  -ms-transform: rotate(-45deg);
  -o-transform: rotate(-45deg);
  transform: rotate(-45deg);

  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  -o-user-select: none;
  user-select: none;
}

Source