What is client-side A/B testing?

Client-side A/B testing runs in the browser using JavaScript. The original page loads first, then the testing script modifies the DOM to show the variant. It's easy to set up but can cause flicker.

What is server-side A/B testing?

Server-side A/B testing determines the variant before the page is rendered. The server sends the correct version directly to the browser. No flicker, but requires developer implementation.

Which is better: client-side or server-side testing?

It depends on your needs. Client-side is easier and doesn't require developers. Server-side is better for performance-critical sites and complex tests. Many teams use both.

Client-Side vs Server-Side A/B Testing: Complete Comparison

TL;DR

Client-side testing runs in the browser (easy setup, possible flicker). Server-side testing runs on the server (no flicker, requires developers). Use client-side for quick UI tests without developers. Use server-side for performance-critical sites, SPAs, or complex logic tests. Many teams use both.

Who this is for

Teams deciding which testing approach to use
Engineers evaluating experimentation architecture
Anyone experiencing flicker with their current tool

How Each Approach Works

Client-Side Testing

1Browser requests page

2Server sends original page

3JavaScript loads and runs

4DOM modified to show variant

⚠️ Risk: Users may see original content before variant loads (flicker)

Server-Side Testing

1Browser requests page

2Server determines variant

3Server sends correct version

4Browser renders page

✓ Benefit: User sees correct variant immediately—no flicker

Feature Comparison

Feature	Client-Side	Server-Side	Winner
Setup Difficulty	Easy (paste snippet)	Complex (code changes)	Client
Developer Required	No	Yes	Client
Flicker/FOOC	Possible	None	Server
Performance Impact	Moderate (100-500ms)	Minimal (<10ms)	Server
Visual Editor	Yes	No	Client
Test Complexity	UI changes only	Any change	Server
SEO Impact	Possible CLS issues	None	Server
Test Pricing	Can test pricing pages	Can test actual prices	Server
SPA Support	Limited	Full	Server
Time to Launch	Minutes	Days/Weeks	Client

When to Use Each Approach

Use Client-Side When...

You don't have developer resources
Testing simple UI changes (text, colors, images)
You need to launch tests quickly
Using website builders (Webflow, Squarespace)
Marketers need to run tests independently

Use Server-Side When...

Performance is critical (Core Web Vitals)
Testing SPAs (React, Vue, Angular)
Testing backend logic (pricing, algorithms)
Flicker is unacceptable
Testing personalization

Performance Impact

Client-Side Impact

• Script size: 50-200KB typical
• Latency added: 100-500ms
• CLS risk: High (if not anti-flickered)
• LCP impact: Moderate

Server-Side Impact

• Script size: 0KB (no client script)
• Latency added: <10ms
• CLS risk: None
• LCP impact: Minimal

💡 Tip: If you use client-side testing, choose a tool with anti-flicker technology (like ExperimentHQ) that hides the page until variants are applied.

The Hybrid Approach (Best of Both)

Many successful teams use both approaches:

Client-side for marketing: Quick UI tests on landing pages, headlines, CTAs—no developer needed.

Server-side for product: Complex tests in the app, pricing experiments, feature rollouts.

ExperimentHQ supports both approaches—use the visual editor for quick tests, or integrate with your backend for server-side experiments.

FAQ

What is flicker in A/B testing?▼

Flicker (or FOOC - Flash of Original Content) is when users briefly see the original page before the variant loads. It happens with client-side testing when the JavaScript takes time to modify the DOM.

Can I use client-side testing with React/Vue?▼

You can, but it's tricky. SPAs re-render the DOM, which can conflict with client-side modifications. Server-side or edge-based testing works better for SPAs.

Is server-side testing more accurate?▼

Not inherently. Both can be equally accurate if implemented correctly. Server-side avoids some edge cases (like users with JavaScript disabled), but these are rare.

Our Recommendation

For most teams: Start with client-side testing. It's faster to set up and doesn't require developers. Use a tool with anti-flicker technology.

For performance-critical sites: Invest in server-side testing. The setup cost is worth it for zero-flicker, SEO-safe experiments.

For mature teams: Use both. Client-side for marketing, server-side for product.