Below are a list of best practices and techniques that you should be using while building out your HTML5 banners (and other websites).
CSS should be clear and concise. You should not have any rules that are unused. You can check for unused styles by running the "Audit" feature found in Chrome's Developer Tools. Try to combine rules if possible to retain wasted bytes.
JS should be clear and concise. Try not to use external libraries at all if possible. Try to leverage recursive functions whenever possible to reduce byte-size.