Senior Developer

The Critical Mass technology team is looking for an inspired, motivated, and experienced Senior Developer to join our Communications team for a contract engagement. We are an experience design agency with a focus on the customer, which means that collaboration and precision execution are at the heart of our process. You'll be working on high-visibility campaign and consumer-facing websites where pixel-perfect design execution and responsive optimization are critical to delivering a premium user experience.

PLEASE NOTE – This is a contract position from July 1 – September 25, 2026

You Will:

• Deliver pixel-perfect front-end implementations that match design comps across all devices and viewport sizes


• Build responsive campaign layouts with modern animation-rich marketing pages using modern HTML, CSS, and JavaScript


• Work on individual components and page sections within robust, complex websites


• Collaborate with Design, UX, and the delivery team to ensure seamless implementation of designs


• Optimize page performance and ensure WCAG accessibility compliance


• Jump into high-velocity production work with minimal onboarding and deliver under tight timelines


• Take ownership of your work and drive solutions proactively with minimal supervision


• Support the team with clear code, documentation, and knowledge sharing

You Have:

• 4+ years demonstrated experience using Vanilla JavaScript


• 5+ years’ experience using HTML5 semantic markup, CSS3, and CSS/SASS preprocessing


• 5+ years building SVG-based animations and responsive consumer-facing websites


• Strong expertise in pixel-perfect design implementation and responsive design across devices


• Full command of Git, the terminal, and modern development fundamentals


• Strong troubleshooting ability and technical analysis skills – including web performance optimization


• Proven ability to jump into projects quickly and deliver quality work under pressure


• Strong communication skills with a self-sufficient, collaborative approach to teamwork


• Fluency with implementing pre-made components from a component library such as Material UI or similar


• Proven knowledge using utility classes, grid classes, and utilizing CSS frameworks such as Bootstrap, Foundation, Materialize, or similar


• Experience with navigating Sketch design files to find font and color styles and dimensions


• Proven track record of delivering production-quality code on high-visibility projects


• Practice or exposure to developing software with Large Language Models (LLMs), including writing and integrating AI-assisted code


• Experience with web animations (CSS and JS) and web animation libraries (GSAP) 

Bonus if you Have:

• Capability with Handlebars or other templating engines


• Skill with Webpack or other front-end build system optimization tools


• Experience working in an agency or fast-paced production environment


• Familiarity with design-to-development handoff processes and design systems


• Awareness with the Atlassian suite of tools (JIRA, Bitbucket, Confluence)


• WCAG accessibility expertise

Critical Mass is an equal opportunity employer. 

Critical Mass uses artificial intelligence in our recruitment process to enhance job postings, filter keywords during the review of prospective candidates, and, in some cases, transcribe interviews with our recruiters. Human review remains central to the process, and all hiring decisions are ultimately made by our team.

The Critical Mass Talent Acquisition team will only communicate from email addresses that use the URLs criticalmass.com, omc.com and us.greenhouse-mail.io. We will not use apps such as Facebook Messenger, WhatsApp, or Google Hangouts for communicating with you. We will never ask you to send us money, technology, or anything else to work for our company. If you believe you are the victim of a scam, please review your local government consumer protections guidance and reach out to them directly.

If U.S. based: https://www.consumer.ftc.gov/articles/job-scams#avoid
If Canada based: https://www.canada.ca/en/services/finance/consumer-affairs.html
If U.K. based: https://www.gov.uk/consumer-protection-rights
If Costa Rica based: https://www.consumo.go.cr/educacion_consumidor/consejos_practicos.aspx