If you would like to disable the text selection highlighting that is enabled by default on all browsers, then you can do this:

user-select: none;

If you are concerned about cross-browser support, then use this class:

.noselect {
  -webkit-touch-callout: none; /* iOS Safari */
    -webkit-user-select: none; /* Safari */
     -khtml-user-select: none; /* Konqueror HTML */
       -moz-user-select: none; /* Old versions of Firefox */
        -ms-user-select: none; /* Internet Explorer/Edge */
            user-select: none; /* Non-prefixed version, currently
                                  supported by Chrome, Edge, Opera and Firefox */

You can then use it like this:

  Selectable text.
<p class="noselect">
  Unselectable text.