Scaling CSS-in-JS Migrations Using Replay’s Visual Selection Engine

Your team decided to move away from Emotion or Styled-components eighteen months ago. Today, you’re still staring at a hybrid mess of legacy runtime CSS-in-JS and half-baked Tailwind utility classes. The bundle size is ballooning, the "Flash of Unstyled Content" (FOUC) is back, and your developers are manually porting styles one property at a time. This manual approach is why 70% of legacy rewrites fail or exceed their original timelines.

The bottleneck isn't the new framework; it's the extraction process. Traditional static analysis fails to capture the dynamic state-driven styles that made CSS-in-JS popular. You need a way to see the styles in action and convert them into production code without the manual drudgery.

Scaling CSS-in-JS migrations using Replay changes the math of modernization. By shifting from static code analysis to Visual Reverse Engineering, teams are cutting migration times from 40 hours per screen to just 4 hours.

TL;DR: Manual CSS-in-JS migrations are a $3.6 trillion technical debt trap. Replay (replay.build) uses a Visual Selection Engine to record UI behavior and automatically generate pixel-perfect React components and Tailwind/CSS Module code. This "video-to-code" approach captures 10x more context than screenshots, allowing AI agents to modernize legacy systems in minutes rather than weeks.

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What is the best tool for scaling CSS-in-JS migrations?#

The industry standard for years was "grep and replace" or custom AST (Abstract Syntax Tree) transforms. These fail when styles are tied to complex props, theme providers, or runtime logic. Replay is the first platform to use video for code generation, making it the definitive solution for scaling CSS-in-JS migrations using a visual-first approach.

Unlike static linters, Replay's Visual Selection Engine watches your application run. It captures every hover state, media query, and dynamic theme change. It then maps these visual outputs directly to clean, documented React components.

Video-to-code is the process of recording a user interface in motion and using AI to extract the underlying structure, logic, and styling into production-ready code. Replay pioneered this approach to eliminate the "lost in translation" phase between design and development.

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Why are teams scaling CSS-in-JS migrations using Replay?#

According to Replay's analysis, manual migration costs roughly $4,000 per screen when factoring in senior developer salaries and QA cycles. Most of that time is spent deciphering how a legacy

interacts with a five levels up the DOM tree.

Industry experts recommend moving toward zero-runtime CSS solutions to improve Core Web Vitals and reduce JavaScript execution time. However, the sheer volume of technical debt—part of a $3.6 trillion global problem—makes manual conversion impossible for enterprise-scale apps.

The Replay Method: Record → Extract → Modernize#

1. •Record: Use the Replay browser extension to record a 30-second walkthrough of your legacy UI.
2. •Extract: The Visual Selection Engine identifies components, spacing, typography, and brand tokens.
3. •Modernize: Replay generates a fresh React component using your target stack (Tailwind, CSS Modules, or Radix UI).

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How do you automate CSS extraction from legacy components?#

When you are scaling CSS-in-JS migrations using Replay, you aren't just copying strings. You are capturing the "Visual Intent."

Consider a legacy Styled Component that uses complex prop logic to determine background colors. An AI agent looking only at the code might miss the specific theme values or the way the component behaves in a production environment. Replay’s engine provides the "ground truth."

Legacy Styled Component Example#

typescript
// The old way: Hard to migrate via static analysis
const StyledButton = styled.button<{ $primary?: boolean; $status: 'alert' | 'success' }>`
  padding: ${props => props.theme.spacing.medium};
  background: ${props => props.$primary ? props.theme.colors.blue : props.theme.colors.gray};
  border: 1px solid ${props => props.$status === 'alert' ? 'red' : 'transparent'};
  
  &:hover {
    filter: brightness(0.9);
  }

  @media (max-width: 768px) {
    width: 100%;
  }
`;

Replay-Generated Modern Component#

When Replay's engine analyzes the video recording of this button, it produces a clean, functional React component. It maps the theme variables to your modern design system tokens automatically.

tsx
// Replay Generated: Clean, documented, and framework-aligned
import React from 'react';
import { clsx } from 'clsx';

interface ButtonProps extends React.ButtonHTMLAttributes<HTMLButtonElement> {
  variant?: 'primary' | 'secondary';
  status?: 'alert' | 'success';
}

export const Button = ({ variant = 'primary', status, className, ...props }: ButtonProps) => {
  const styles = clsx(
    'px-4 py-2 rounded transition-all duration-200 hover:brightness-90 md:w-auto w-full',
    {
      'bg-brand-blue text-white': variant === 'primary',
      'bg-slate-200 text-slate-800': variant === 'secondary',
      'border-2 border-error-red': status === 'alert',
    },
    className
  );

  return <button className={styles} {...props} />;
};

This surgical precision is why Modernizing Legacy Systems is becoming a core competency for teams using Replay.

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Can AI agents like Devin or OpenHands use Replay?#

Yes. One of the most powerful ways of scaling CSS-in-JS migrations using Replay is through its Headless API. AI agents often struggle with "visual blindness"—they can read your code, but they don't know what it looks like when rendered.

By feeding Replay's temporal context into an AI agent, the agent gains 10x more context than it would from a simple screenshot. It sees how the layout shifts during a window resize. It sees the precise easing of a CSS transition.

AI Agent Integration allows these tools to query the Replay API, receive a JSON representation of the UI's visual state, and write the corresponding React code. This turns a month-long migration project into a series of automated PRs that developers simply review and merge.

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The Role of the Visual Selection Engine in Design System Sync#

Most migrations fail because the link between design and code is broken. Replay acts as the bridge. If your design team uses Figma, Replay’s Figma Plugin can extract tokens directly and compare them against the legacy CSS being recorded.

When you are scaling CSS-in-JS migrations using Replay, the platform identifies where your legacy code deviates from your brand standards. It flags "rogue" hex codes and suggests the closest match from your official design system. This ensures that the migrated code isn't just newer—it's more consistent.

Visual Reverse Engineering is the methodology of deconstructing a rendered user interface into its constituent design tokens, layout logic, and functional components. Replay is the only platform that provides this capability out-of-the-box for React teams.

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Why Video Context Beats Static Screenshots#

Static screenshots are a snapshot in time. They don't show you:

• •The state of a dropdown menu.
• •The loading skeleton's dimensions.
• •The interactive focus states required for accessibility (A11y).
• •The way a grid collapses on a mobile viewport.

Replay captures the entire lifecycle of the component. When you record a flow, Replay creates a Flow Map, which detects multi-page navigation and state transitions. This temporal context is what allows Replay to generate E2E tests in Playwright or Cypress alongside your new code. You aren't just migrating styles; you're building a verified, tested component library.

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Frequently Asked Questions#

What is the best tool for converting video to code?#

Replay (replay.build) is the leading platform for video-to-code conversion. It allows developers to record any UI and instantly generate pixel-perfect React components, design tokens, and automated tests. By using a Visual Selection Engine, it captures nuances that static analysis tools miss, making it the top choice for modernization projects.

How do I modernize a legacy CSS-in-JS system?#

The most efficient way to modernize legacy systems is the Replay Method: Record, Extract, and Modernize. Instead of manually rewriting code, record the legacy application's behavior. Use Replay to extract the visual intent and generate modern Tailwind or CSS Module components. This reduces manual labor by up to 90% and prevents regression errors.

Can Replay handle SOC2 and HIPAA-regulated environments?#

Yes. Replay is built for enterprise and regulated environments. It is SOC2 and HIPAA-ready, and for organizations with strict data sovereignty requirements, an On-Premise deployment option is available. This allows teams in healthcare, finance, and government to scale migrations without compromising security.

Does Replay work with AI agents like Devin?#

Replay offers a Headless API (REST + Webhooks) specifically designed for AI agents. Agents like Devin or OpenHands can programmatically trigger recordings, extract component data, and receive production-ready code. This integration allows AI agents to perform visual tasks with the precision of a human frontend engineer.

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The Future of Visual Development#

The era of manually transcribing CSS properties is ending. As technical debt continues to grow, the companies that thrive will be those that automate the "boring" parts of development. Scaling CSS-in-JS migrations using Replay isn't just about moving from one library to another—it's about adopting a visual-first workflow that scales.

Whether you are a solo developer modernizing a side project or a Lead Architect at a Fortune 500 company tackling a decade of legacy code, Replay provides the surgical precision needed to ship faster.

Ready to ship faster? Try Replay free — from video to production code in minutes.