I recently updated my Base64 Decoder/Encoder, JSON Pretty Print and XML Pretty Print utilities by adding a button that copies the text to the clipboard. The technique I used is to select the field containing the text, and then call document.execCommand('copy') to copy the field's text into the clipboard. The following code snippet first selects the <TEXTAREA> with an id of "Text", and then copies its text to the clipboard. It uses jQuery for the necessary client-side HTML manipulation.

Note, document.execCommand('copy') doesn't work on every browser, but I've verified that it works on Firefox, IE, and Chrome on the desktop, and on the Chrome browser on Android. That's why the call is inside of a try / catch block.

$(document).ready(function () {
    $('#copy').click(function() {
        $('#Text').select();

        try {
            var successful = document.execCommand('copy');
        } catch (err) {
            console.log('unable to copy: ' + err);
        }
    });
});

No-one can copy my good looks!