Old Android phones and poor camera phones does not scanning WhatsApp Web QR code. My 5 year old HTC wildfire continuously scanning the QR code, but not logging in my browser. Then this trick worked to use whatsapp Web client in my laptop browser. Any browser Chrome or firefox. Make sure your mobile and pc connected to working internet. If scanning didnt work normally. Then try the steps in video. Hope this trick will work for you. Comment if it worked.