Other solutions are great but they didn't take care of the fact that watermark shouldn't get selected on selection from the mouse. This fiddle takes care or that: https://jsfiddle.net/MiKr13/d1r4o0jg/9/
This will be better option for pdf or static html.
CSS:
#watermark {
opacity: 0.2;
font-size: 52px;
color: 'black';
background: '#ccc';
position: absolute;
cursor: default;
user-select: none;
-webkit-user-select: none;
-khtml-user-select: none;
-moz-user-select: none;
-ms-user-select: none;
right: 5px;
bottom: 5px;
}