How do I disable text selection with CSS or JavaScript?

<div 
 style="-moz-user-select: none; -webkit-user-select: none; -ms-user-select:none; user-select:none;-o-user-select:none;" 
 unselectable="on"
 onselectstart="return false;" 
 onmousedown="return false;">
    Blabla
</div>

UPDATE January, 2017:

According to Can I use, the user-select is currently supported in all browsers except Internet Explorer 9 and earlier versions (but sadly still needs a vendor prefix).

All of the correct CSS variations are:

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

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

Note that it's a non-standard feature (i.e. not a part of any specification). It is not guaranteed to work everywhere, and there might be differences in implementation among browsers and in the future browsers can drop support for it.

More information can be found in Mozilla Developer Network documentation.

You can use JavaScript to do what you want:

if (document.addEventListener !== undefined) {
    // Not IE
    document.addEventListener('click', checkSelection, false);
} else {
    // IE
    document.attachEvent('onclick', checkSelection);
}

function checkSelection() {
    var sel = {};
    if (window.getSelection) {
        // Mozilla
        sel = window.getSelection();
    } else if (document.selection) {
        // IE
        sel = document.selection.createRange();
    }

    // Mozilla
    if (sel.rangeCount) {
        sel.removeAllRanges();
        return;
    }

    // IE
    if (sel.text > '') {
        document.selection.empty();
        return;
    }
}

Soap box: You really shouldn't be screwing with the client's user agent in this manner. If the client wants to select things on the document, then they should be able to select things on the document. It doesn't matter if you don't like the highlight color, because you aren't the one viewing the document.

Try this CSS code for cross-browser compatibility.

-webkit-user-select: none;
-khtml-user-select: none;
-moz-user-select: none;
-o-user-select: none;
user-select: none;