tab-focus.js

// ==========================================================================
// tab-focus.js
// Detect keyboard tabbing
// ==========================================================================

(function() {
    var className = "tab-focus";

    // Remove class on blur
    document.addEventListener("focusout", function(event) {
        event.target.classList.remove(className);
    });

    // Add classname to tabbed elements
    document.addEventListener("keydown", function(event) {
        if (event.keyCode !== 9) {
            return;
        }

        // Delay the adding of classname until the focus has changed
        // This event fires before the focusin event
        window.setTimeout(function() {
            document.activeElement.classList.add(className);
        }, 0);
    });
})();