Front-End CRO Web Developer

This is Gain Ltd; Greater London; Hybrid; GBP 40,000 - 60,000; Full time

Job summary

A leading digital performance agency in Greater London is looking for an experienced web developer to support clients' experimentation programs. The role requires strong skills in HTML, CSS, and JavaScript, along with collaboration across various teams. Ideal candidates have 3-5 years of experience and are able to manage multiple client projects simultaneously. The agency fosters a supportive and inclusive culture with a hybrid work environment and competitive benefits.

Benefits

Hybrid work environment

Private Medical Insurance

BUPA Life Assurance

Employee Assistance Program

Qualifications

• 3-5 years of professional experience with front-end technologies.
• Excellent knowledge of native JavaScript.
• Experience with A/B testing platforms or ability to learn tools rapidly.
• Proficient in design tools for development implementation.

Responsibilities

• Build client-side experiments using HTML, CSS, and JavaScript.
• Work with experimentation platforms to implement tests.
• Conduct final reviews to ensure coding standards.
• Collaborate cross-functionally with teams to achieve goals.

Skills

HTML

CSS

JavaScript

Problem-solving skills

DOM manipulation

Tools

GitHub

Figma

Google Analytics

Job description

Welcome to Gain Conversion!

Gain is a creative-led, insight-driven company that blends data, tech and creativity. We believe the best ideas emerge where intelligence and creativity unite, where insight sparks imagination, and where innovation turns possibility into progress. We are explorers of new frontiers, shaping bold strategies that move people, brands, and businesses forward. Individually and together, our specialist teams provide the vision, data, and confidence brands and organisations need to make braver, more impactful decisions.

At Conversion, we’re a team of global CRO experts who help brands optimise their digital ecosystems, websites, platforms, and customer experiences to drive real, measurable growth. We intend to prove that evidence-based decisions get the best results, and our people and unique culture are the fuel. As a certified Great Place to Work®, we support each other fully, without ego or drama, and stay focused on what’s best for our clients and each other. We’re proud to be an inclusive company where people can bring their whole selves to work. Our guiding principles - Be One, Be Decisive, Be Hungry, Be Undeniable, and Be Accountable - are at the core of everything we do. Here, you’ll work with talented, passionate people who challenge you to do your best every day.

If this sounds like an adventure you’d like to be part of - then keep reading and apply below!

The Role

As a web developer with strong HTML / CSS / JavaScript skills, you will support our clients' experimentation programmes by building client-side A / B test variations. Ideally, you already have experience with client-side CRO platforms (e.g. Optimizely Web, Kameleoon, Convert, AB Tasty, VWO etc.) and can hit the ground running. If not, you're an excellent web developer who learns fast and thrives in a fast-paced agency environment.

Responsibilities

• Build client-side experiments to exact specification using HTML, CSS, and JavaScript
• Work with experimentation platforms to implement and deploy tests
• Oversee and conduct final reviews to ensure experiment designs are being followed and coding standards are maintained
• Review code from other developers in the development team
• Prioritise and fix bugs promptly
• Perform technical evaluations to determine if testing scripts will impact existing code
• Stay up to date with releases and updates from A / B testing platforms that impact how the development team works with clients
• Handle other ad-hoc requests and support the delivery team
• Working as a member of a larger team and collaborating effectively with others in your function (developers) as well as cross functionally (designers, project managers, consultants, QA engineers, testers and clients) to support each other, achieve the best outcomes, and win as a team
• When required, participate in client calls to discuss technical feasibility, timelines, and troubleshooting
• Work within a portfolio model, owning relationships and delivery for 5-10 clients simultaneously

Requirements

The ideal candidate has the following qualifications, but we encourage you to apply even if you feel you do not meet 100% of the requirements below.

• 3-5 years professional experience with front-end technologies: HTML, CSS, JavaScript
• Excellent knowledge of native JavaScript and strong problem-solving skills
• Experience with DOM manipulation and client-side script injection
• Experience with A / B testing platforms or demonstrable ability to learn technical tools rapidly
• Proficient in design tools (e.g. Figma preferred) for development implementation: asset extraction, CSS generation, and element measurement
• Proficient use of browser developer tools and console
• Understanding of how SPA frameworks (React, Angular, Vue) work - able to inject client-side code without breaking React / Vue applications
• Experience in cross-browser / device testing and ensuring consistency
• Experience with version control such as GitHub, Bitbucket
• An Agile / Scrum developer with 3+ years of experience in an agency or startup environment

Nice to haves

• Experience with analytics tools such as Google Analytics, Adobe Analytics
• Hands‑on experience with SPA frameworks (React, Angular, Vue)
• Experience with JavaScript build tools such as Webpack, Vite
• Experience with back‑end technologies: Node, Express, PHP, Laravel
• Knowledge of RESTful Services and APIs
• Experience with Command Line Interface (CLI) tools
• Experience with server‑side / full‑stack A / B testing tools such as Optimizely Feature Experimentation, Amplitude, LaunchDarkly
• Experience with AI development tools
• Experience with DevOps using AWS
• Experience with Jira

Behaviours

• Self‑starter who takes ownership of deliverables without needing hand‑holding
• Resourceful problem‑solver and fast learner who finds solutions before escalating issues
• Eye for detail - spot subtleties or mistakes that others haven't and ensure pixel‑perfect conversion of designs into HTML / CSS
• Productive and efficient - able to build experiments quickly without sacrificing quality
• Strong time management and ability to effectively manage multiple client projects simultaneously while meeting deadlines, collaborating cross‑functionally with designers, project managers, consultants, and clients
• Technologically savvy, always meeting and exceeding high standards of accuracy and quality
• Able to identify technical problems and develop sound, carefully thought‑out solutions
• Proactive approach to clients and workload management
• Positive attitude to challenges and change
• A firm, factual communicator, able to complete quick decisions in response to changing conditions
• Balanced approach of what's right for the client vs what's commercially right for the business

Benefits

• Hybrid work environment
• Private Medical Insurance
• BUPA Life Assurance
• Income protection
• Employee Assistance Program
• Cycle to Work salary sacrifice scheme
• Tech & Wearables salary sacrifice scheme
• Octopus EV Scheme
• Discounts and deals on a range of items from hotels, holidays and hormone testing to cinema, gyms and will writing

GAIN is an equal opportunities employer and positively encourages applications from suitably qualified and eligible candidates regardless of sex, race, disability, age, sexual orientation, gender reassignment, religion or belief, marital status, or pregnancy and maternity. We believe that diversity and inclusivity make our teams better, so it is our priority to create an equitable and inclusive workplace where our goals, initiatives, and commitments to action are shaped and strengthened by our team’s voices. Putting People First, one of our core values, means we never lose sight of humanity, care, and impact.

Disclaimer : we never request any form of payment or fees from candidates at any stage of the recruitment process. If you are contacted by anyone claiming to represent our company and asking for money or personal financial details, please report it immediately to [e-mail]. All genuine communication will come from official company channels.