Note: After saving, you have to bypass your browser's cache to see the changes. Google Chrome, Firefox, Microsoft Edge and Safari: Hold down the ⇧ Shift key and click the Reload toolbar button. For details and instructions about other browsers, see Wikipedia:Bypass your cache.
a:hover[accesskey]:before {
    content: " " attr(accesskey) " ";
    text-transform: uppercase;
    white-space: pre;
    font-family: sans-serif;
    margin-right: 0.5ex;
}