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Client-Side Testing

What is client-side testing?

Client-side testing is the experimentation process of rendering variations at the browser level. 

When a user visits a webpage on a browser, it initiates a request to the server, which sends the required file to the browser. The browser then processes the file and displays the webpage. The interactive elements of the webpage are made possible through the use of JavaScript within the files. 

JavaScript is also used to generate variations for experimentation like A/B tests, multivariate tests, and split tests. The JavaScript can be run at the browser level or the server itself. The JavaScript runs at the browser level for client-side testing, while the process of rendering the variations on the server is called server-side testing.

A use case of client-side testing is delivering call-to-action (CTA) button variations for an A/B test. Once a user becomes part of the A/B test, the DOM selectors identify the HTML element (in this case, the CTA button), then execute the JavaScript at the browser to modify CTA and create variation. The modification can be anything like changing CSS style, hiding, removing, resizing, and duplicating. 

Advantages of client-side testing

Here are the benefits of client-side testing:

Easy to adopt

There are client-side A/B testing tools like VWO that offer a WYSIWYG editor, which doesn’t require coding to create variations. Such testing frameworks make it easy to adopt client-side testing, especially if the company is small and can’t afford a development team.  

Low investment cost

It requires less financial investment compared to server-side testing. Client-side testing is a preferred mode of experimentation for businesses that are on a budget crunch or new to optimization.

Ability to test often

Client-side testing allows for intuitive tools to create variations, which makes it easy for the CRO team to create variations frequently and test more hypotheses.

Limitations of client-side testing

Here are the limitations of client-side testing:

May reduce the website speed

The JavaScript runs at the browser level, which increases the LCP (Longest contentful paint) time compared to server-side rendering.  


In client-side testing, a flash of original content appears before the variations, which is called the flicker effect. It may occur when the browser loads the variations by running JavaScript. 

Limited to UI/UX testing

The JavaScript runs at the browser end, which is why there is a limit to the scope of experimentation. Complex experimentation like product search algorithm test is not feasible with client-side testing. Thus, complex and dynamic experimentation is possible only via server-side testing. 

Cannot run omnichannel experiments

Client-side tests cannot run on the web and app at the same time. For omnichannel experiments, server-side experimentation is suitable.

Lifecycle of the client-side testing

Client-side testing involves multiple stages, which are as follows:

a. Behavior analysis

Optimization starts with understanding the existing user flow and behavior. Typically, behavior analysis involves studying visitor data on analytics tools, analyzing dynamic heatmaps, understanding funnel journeys, and watching session recordings. 

b. Creating hypothesis

Based on the behavior analysis, a hypothesis that highlights the element for experimentation and the expected effect on a KPI is formulated. 

c. Client-side testing

Client-side A/B or multivariate test is conducted to track and compare the performance of the variation to the original page called control. The experimentation ends with a winner. One important thing to note here is that multi-armed bandit testing is gaining popularity because it exposes the traffic to better-performing variation with time before the test reaches statistical significance. 

d. Deploy

In the end, the winning variation is deployed on the live website exposing all the visitors to the new experience. 

e. Documentation

The whole process of client-side testing is documented with learnings, rectified mistakes, and results. It acts as a reference for future optimization campaigns. 


To summarize, client-side testing involves rendering experimentation variations at the browser level using JavaScript to modify elements like call-to-action buttons. It is easy to adopt, has low investment cost, and allows for frequent testing, but it can reduce website speed, causing flickering, and is limited to UI/UX testing. 

The lifecycle of client-side testing involves behavior analysis, creating a hypothesis, client-side testing, and deployment of the winning variation. Complex and dynamic experimentation is feasible only via server-side testing.
You can start client-side testing with VWO for free. Know more about the VWO Testing starter pack on the plans and pricing page.