Best practices

Good tests rarely fail on technology — they fail on a fuzzy hypothesis, too short a runtime, or too many parallel changes. The patterns below keep planning and implementation sharp.

Hypothesis → KPI → runtime

Hypothesis
Write a testable statement: “If we change the CTA copy to ‘Start free trial’, signup rate will increase because the entry barrier is clearer.”
KPI / primary goal
One primary event (e.g. signup). Secondary metrics (bounce, revenue) are context only — not a post-hoc “winner hunt.”
Runtime & sample size
Plan at least 1–2 weeks or until minimum sample size per variant is reached (default: 100 sessions). Account for seasonal swings (weekdays!).
One change per test
Otherwise you will not know what caused the effect. Split large redesigns into sequential tests.
Decision
Roll out only with significance and adequate sample size — see Analytics & evaluation.

Test planning and implementation

Clear hypothesis: Define what you want to test
Measurable goal: Determine how you will measure success
Sufficient duration: Plan for at least 1–2 weeks
Statistical significance: Wait for enough data

/* Good practice: Specific selectors */
.ab-test-variant-1 .hero-button {
  background-color: #10b981;
  font-weight: bold;
}

/* Avoid: Too broad selectors */
/* button { background-color: #10b981; } */

Broad selectors leak into other page areas and make debugging harder.

// Good practice: Defensive programming
if (document.querySelector(".target-element")) {
  document.querySelector(".target-element").textContent = "New Text";
}

// Good practice: Error handling
try {
  // Variant-specific code
  initVariantFeature();
} catch (error) {
  console.warn("A/B test variant could not be initialized:", error);
}

Errors in variant JS should not block the rest of the page.

SEO and flicker

SEO

Keep both variants meaningful in content (no keyword-spam version “just for the test”).
Avoid search engines indexing conflicting hero copy long-term: keep tests short, then ship the winner in markup.
Purely visual CSS changes (color, spacing) have low SEO risk; for headline/content swaps, prefer HTML variants with a clear control as fallback.

Flicker (FOUC)

Always set a fallback element.
Optionally hide the body briefly until the variant is applied — recipe and debugging: Troubleshooting → Flicker effect.
Load the script as early as practical in <head> (defer is fine; blocking only if you must hard-avoid flicker).