How to Run A/B Tests Without Flicker

TL;DR

To eliminate flicker: (1) Use lightweight tools (<10KB scripts), (2) Load scripts synchronously in <head>, (3) Consider server-side testing for critical pages. Anti-flicker snippets hide flicker but add perceived latency. The best solution is using tools that are fast enough to not need workarounds.

What is A/B Test Flicker?

Flicker is when users briefly see the original page before the variant loads. It happens because:

1. Browser renders the original HTML
2. A/B testing script loads (takes time)
3. Script modifies the DOM to show variant
4. User sees a "flash" of the original content

Anti-Flicker Techniques

Technique	Effectiveness	Complexity	How it Works
Synchronous loading	Good	Low	Load script in <head> before page renders
Anti-flicker snippet	OK	Low	Hide page until variant loads
Server-side rendering	Best	High	Render variant on server
Lightweight scripts	Good	None	Use tools with small script size

Method 1: Synchronous Loading

Load the A/B testing script synchronously in the <head> before any content renders:

<head>
  <!-- Load A/B testing script FIRST, synchronously -->
  <script src="https://cdn.experimenthq.io/snippet.js"></script>
  
  <!-- Other scripts load after -->
</head>

Trade-off: Blocks rendering until script loads. Only works well with small scripts (<10KB).

Method 2: Anti-Flicker Snippet

Hide the page until the variant is ready:

<style>
  .async-hide { opacity: 0 !important; }
</style>
<script>
  document.documentElement.classList.add('async-hide');
  // Remove class when variant is ready (max 2 seconds)
  setTimeout(() => {
    document.documentElement.classList.remove('async-hide');
  }, 2000);
</script>

Trade-off: Page appears blank until variant loads. Can feel slow.

Method 3: Server-Side Testing

The only true zero-flicker solution. Render the variant on the server before sending HTML:

// Server-side (Node.js example)
const variant = experimenthq.getVariant(userId, 'experiment-id');

if (variant === 'control') {
  return renderControlPage();
} else {
  return renderVariantPage();
}

Trade-off: Requires code changes and server-side implementation.

Our Recommendation

Easiest solution: Use ExperimentHQ. Our <5KB script is fast enough that flicker is imperceptible without any workarounds.

For critical pages: Consider server-side testing with tools like Statsig or LaunchDarkly.