Banner ads: the kilobyte conundrum

October 15th 2015 | GreenSock


Banner ads: the kilobyte conundrum

In recent months, the whirlwind shift to HTML5 in the banner ad industry has prompted a slew of policy changes. Publishers and networks are scrambling to answer questions from designers who want to build things "properly" (a term whose meaning can vary wildly these days). Growing pains abound.

Shortly after we published an article describing the state of affairs and offering recommendations for a path forward, the IAB released a draft of their new HTML5 ad specs which echoed many of those recommendations. For example, the file size limit for standard ads was raised to 200kb. This was cause for much celebration across the industry.

But we're not out of the woods yet. There is still a persistent mindset about how we look at kilobytes that's crippling the web. This article aims to challenge the old paradigm and explain why it's so important to re-assess kilobyte costs.

Why limit kilobytes?

Conventional wisdom says that kilobytes have a direct impact on load times and consequently user experience. File size limits exist to promote better performance. Period. Does conventional wisdom apply the same way in the HTML5 era? As we modernize our kilobyte-count policies, let's remember what the goal is: performance...or more accurately, better user experience.

Not all kilobytes are created equal

HTML5 has unique strengths that challenge us to move beyond the simplistic "aggregate total file size" mentality of yesteryear. We need to look at kilobyte cost in a new, more nuanced way. There are four primary factors:

  1. Cache status

    When is 35kb not really 35kb? When it's cached.

    A cached file has absolutely zero bandwidth cost regardless of its size. It loads immediately. This is particularly relevant in banner ads because there are certain chores common to almost every ad (like animation management tasks) that can be encapsulated and shared among many, many banners. The end user only loads that shared resource once and then it's cached and completely "free" thereafter...for all ads pointing at that file...on all sites.

    Does it really make sense to penalize ads for using those ubiquitous shared resources even though they're so pervasively cached that they don't typically affect load performance? In the modern world, file size limits should apply to the banner-specific assets that have a direct impact on loading times, not standardized shared resources.

  2. Location

    Kilobytes loaded from a CDN (Content Delivery Network) are typically "faster" because they're geographically dispersed and automatically loaded from the closest server. Plus CDNs have inherent redundancies leading to better reliability.

  3. Spread

    200kb spread across 24 files will take longer to download than 200kb spread across only 4 files. This isn't particularly relevant in the discussion about shared resources (which should be cached very quickly), but is an argument in favor of loading TweenMax rather than the combination of TweenLite, CSSPlugin, EasePack, and TimelineLite even though they collectively use fewer kilobytes. If the industry remains focused solely on aggregate total file size, it pushes designers/developers toward less capable solutions that use fewer kilobytes even though they don't necessarily affect load times and could be replaced by more robust options that allow more creative expression.

  4. Performance yield

    Some kilobytes are cheap in terms of the initial load but expensive for runtime execution. If our goal is better user experience, this factor should weigh heavily into the overall kilobyte cost equation.

    Would you rather have a banner that loads 200ms faster with janky animation or one that's super-smooth at runtime but takes a fraction of a second longer to load? Are publishers primarily concerned with displaying ads faster initially or ensuring that their site runs smoothly once loaded? Of course there are reasonable limits either way (waiting an extra 30 seconds for a huge file to load would be intolerable even if it made things run buttery smooth), but in most cases we're only talking about fractions of a second difference.

    For example, GSAP contains "extra" code that automatically GPU-accelerates transforms, applies lag smoothing, leverages 3D transform matrices, avoids layout thrashing, organizes things internally to make auto overwriting super fast, etc. - would removing those features for the sake of milliseconds on initial load (and zero savings once it's cached) be a step forward or backward?

Incentivizing the wrong things

If the IAB and publishers don't embrace a common set of shared resources that get excluded from file size calculations...

  • It creates inefficiencies and redundancies - thousands of ads may each contain their own [duplicate] copy of a library like TweenMax, squandering valuable bandwidth.
  • It penalizes the use of robust, industry-standard libraries in favor of custom JS and micro-libraries that probably aren't nearly as capable, well-tested, cached, compatible, or performant.
  • When something breaks, it will be more difficult for the various ad networks and publishers to troubleshoot and support custom JS and diverse micro-libraries. Lots of APIs to learn (or in the case of custom JS, more advanced expertise would be required).
  • GreenSock would likely need to create a minimalistic version of TweenMax that has a tiny subset of the features. We feel strongly that this is a step backward and explain why here.

So ultimately, an "all-inclusive" file size policy could actually hurt load times as well as runtime performance. Yet the primary goal of file size limits is to protect performance. Hm.

If the industry rallies behind a few popular, well-maintained and performant libraries, exempting them from file size calculations because of their ubiquity, it would not only deliver a better overall user experience, but also make it easier to create high quality banners. There would be fewer headaches for networks and publishers too.

TweenMax (GSAP's largest file) technically weighs around 34kb but those kilobytes are the remarkably inexpensive kind. TweenMax is widely cached, it's on multiple CDNs, it packs various tools into a single file (zero "spread"), and it has an extremely high performance yield because of its many runtime performance optimizations. There are so many ads using it already that it has little or no effect on load times. Should it really cost banner ad designers/developers 34kb against their file size budgets? Is it wise to incentivize cooking up their own custom code for handling animation tasks instead?

The good news

Every major ad network we’ve contacted understands the value of shared resources and is very GSAP-friendly. In fact, virtually all of them have GSAP on their own CDNs and don't count its file size against ads unless the publisher insists otherwise (which is rare). One notable exception is Adwords, but we have been told they're working on a solution.

  Allows GSAP Excludes GSAP from
file size calculation*
Google DoubleClick YES YES YES
Flashtalking YES YES YES
Cofactor YES YES YES
*Unless publisher objects which is uncommon

Google DoubleClick is even pioneering a process that will automatically detect when GSAP is used in an ad and swap in its own CDN link to maximize caching benefits. Pretty cool stuff.


Let's embrace the unique strengths of HTML5 and modernize the way we count kilobyte costs. Let's support policies that incentivize better performance and user experience rather than a race to the smallest total file size for each individual ad. Caching, CDNs, kilobyte spread, and performance yield should all factor into the way we view kilobytes the HTML5 era.


  1. Isn't it unfair if the IAB only recommends a few popular libraries? What about newer or lesser-known libraries?
    This is an entirely valid concern. The list should be reviewed regularly and the IAB can assess each library's industry support, performance profile, compatibility, and track record for ongoing updates and bug fixes. New contenders could be submitted for consideration anytime. But remember that the key to realizing the performance benefits is keeping the list short so that caching is focused and pervasive. If there are too many "standardized" libraries, it dilutes caching and defeats the purpose. There's no way that everyone will agree on which libraries should be on the list but if we get hung up on not offending anyone or being afraid to appear biased, we'll miss the opportunity to move the industry forward. The list won't be perfect, but not having a list at all is much worse.
  2. What happens when a library gets updated? Wouldn't it need to be re-cached?
    Yes. And trust me - we want libraries to be updated somewhat regularly to work around new browser inconsistencies, patch library bugs, and implement new features that drive things forward. But these updates wouldn't need to interfere with existing or legacy ads - when a library is updated, a new CDN URL would be generated and new ads could optionally point to that version. Those ads would indeed trigger browsers to cache that new version but this should happen very quickly. Most likely within a matter of days the new version would be pervasively cached across the web. Yes, each end user would pay that kilobyte tax once on the first load and then it would be "free" thereafter.
  3. Would GreenSock still recommend this policy if GSAP wasn't included in the short list of exempt libraries?
    Absolutely. This isn't just about getting GSAP an exemption - this is what we believe is best for the industry overall even if GSAP isn't on the list.


Version: 1.19.0 updated 2017-01-17




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