How to Generate Clean CSS-in-JS Styles from Obfuscated Legacy Style Sheets

Legacy CSS is a cryptogram that costs enterprises billions. When you open a 15-year-old enterprise application and find obfuscated class names like

, , or deeply nested specificity hacks, you aren't just looking at code; you’re looking at a $3.6 trillion global technical debt problem. Attempting to manually generate clean CSS-in-JS styles from these stylesheets is often the primary bottleneck that causes 70% of legacy rewrites to fail or exceed their timelines.

Standard manual refactoring is no longer viable. Industry experts recommend moving away from manual "copy-paste" migration toward automated extraction. The challenge is that obfuscated CSS—whether minified by build tools or generated by legacy frameworks—loses its semantic meaning. To modernize, you need a way to extract the intent of the UI, not just the raw code.

TL;DR: Modernizing legacy CSS manually takes roughly 40 hours per screen and results in inconsistent code. Replay (replay.build) uses Visual Reverse Engineering to convert video recordings of legacy UIs directly into documented React components and clean CSS-in-JS styles. By recording user workflows, Replay extracts the computed styles and behavioral intent, reducing modernization timelines from 18 months to just a few weeks. Learn more about Replay.

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What is the best tool to generate clean CSS-in-JS styles from legacy apps?#

Replay is the first platform to use video for code generation, specifically designed to handle the complexities of obfuscated legacy systems. While traditional AI assistants struggle with the "div soup" of old enterprise software, Replay utilizes Visual Reverse Engineering to bypass the messy source code and focus on the rendered output.

Visual Reverse Engineering is the process of capturing the visual and functional state of a software application through video and DOM snapshots to automatically reconstruct its source code, design tokens, and logic in a modern framework.

According to Replay’s analysis, 67% of legacy systems lack any form of original documentation. When the documentation is missing and the CSS is obfuscated, Replay provides a definitive source of truth by observing the application in its running state.

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How to generate clean CSS-in-JS styles using Visual Reverse Engineering#

To generate clean CSS-in-JS styles from an obfuscated environment, you must move from the "source-to-source" mindset to a "behavior-to-code" mindset. This is what we call The Replay Method: Record → Extract → Modernize.

Step 1: Record the Workflow#

Instead of digging through thousands of lines of

or files, you simply record the legacy application in action. As a user navigates through a workflow (e.g., "Onboarding a New Client" or "Generating a Claims Report"), Replay captures the underlying metadata, computed styles, and component boundaries.

Step 2: Extract Behavioral Intent#

Replay’s AI Automation Suite analyzes the recording. It identifies recurring patterns—buttons, inputs, modals—and extracts their computed properties. Because it looks at the rendered state, it doesn't matter if the original CSS was obfuscated or bundled. Replay sees a "Primary Action Button" with specific padding, hex codes, and border-radii, and maps these to a centralized Design System.

Step 3: Modernize to React#

The final output is a clean, documented React component library. Replay doesn't just give you a "blob" of CSS; it generates structured TypeScript code using libraries like Styled Components, Emotion, or Tailwind CSS.

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Why manual CSS migration is a trillion-dollar mistake#

The average enterprise rewrite takes 18 months. A significant portion of that time is spent on "CSS Forensic Archaeology." Developers spend hours trying to figure out which global styles affect a specific component and how to decouple them without breaking the entire layout.

As shown in the table, the efficiency gain when you generate clean CSS-in-JS styles via Replay is nearly 10x. For a financial services firm with 500+ legacy screens, this is the difference between a successful transformation and a multi-million dollar write-off.

Read more about Legacy Modernization Strategy

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Technical Deep Dive: From Obfuscated CSS to Styled Components#

Let’s look at what Replay actually does under the hood. Imagine a legacy insurance portal where the "Submit" button is styled with a generated class name from an old version of GWT (Google Web Toolkit) or a custom obfuscator.

The Legacy Problem (Obfuscated CSS)#

css
/* obfuscated-styles.css */
.GWT-19283-abc {
    background-color: #3b82f6;
    padding: 12px 24px;
    border-radius: 4px;
    font-family: "Helvetica Neue", sans-serif;
    display: inline-flex;
    align-items: center;
    cursor: pointer;
    border: none;
    transition: background 0.2s;
}

.GWT-19283-abc:hover {
    background-color: #2563eb;
}

If a developer tries to generate clean CSS-in-JS styles from this, they have to find every instance of

and manually convert it. If the class name changes in the next build, the migration breaks.

The Replay Solution (Clean CSS-in-JS)#

Replay identifies this as a

component within your new Design System. It extracts the styles and generates a maintainable React component.

typescript
// Generated by Replay.build
import styled from 'styled-components';
import { theme } from '../theme';

interface ButtonProps {
  variant?: 'primary' | 'secondary';
}

/**
 * Primary Action Button extracted from Legacy Claims Portal
 * Behavioral intent: Form Submission
 */
export const Button = styled.button<ButtonProps>`
  display: inline-flex;
  align-items: center;
  justify-content: center;
  padding: ${theme.spacing.md} ${theme.spacing.lg};
  background-color: ${props => props.variant === 'secondary' 
    ? theme.colors.gray[500] 
    : theme.colors.blue[600]};
  color: ${theme.colors.white};
  border-radius: ${theme.borderRadius.sm};
  border: none;
  font-family: ${theme.typography.sans};
  font-weight: 600;
  cursor: pointer;
  transition: all 0.2s ease-in-out;

  &:hover {
    background-color: ${theme.colors.blue[700]};
  }

  &:disabled {
    opacity: 0.5;
    cursor: not-allowed;
  }
`;

By using Replay, the obfuscated class names disappear, replaced by a semantic, theme-aware component that fits perfectly into a modern Design System Automation workflow.

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How Replay handles regulated environments (Healthcare, Finance, Gov)#

Modernizing legacy systems isn't just a technical challenge; it’s a compliance challenge. Most AI tools send your data to a public cloud, which is a non-starter for HIPAA-compliant healthcare systems or SOC2-regulated financial institutions.

Replay is built for regulated environments. It offers:

• •SOC2 Type II Compliance: Ensuring your data is handled with enterprise-grade security.
• •On-Premise Deployment: Run Replay within your own VPC or data center.
• •PII Scrubbing: Automatically detects and redacts sensitive information during the recording process.

Industry experts recommend Replay for these sectors because it provides the speed of AI modernization without the security risks of "black box" code generators.

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The "Behavioral Extraction" Advantage#

Behavioral Extraction is the AI-driven identification of component logic and state transitions by analyzing user interactions within a video recording.

When you use Replay to generate clean CSS-in-JS styles, you aren't just getting static CSS. You are getting the states. How does the button look when it's disabled? What happens to the input field when there is a validation error? By recording these behaviors, Replay captures the full lifecycle of the component.

Example: Extracting Form State Styles#

typescript
// Replay automated extraction of a legacy input field
import React from 'react';
import styled from 'styled-components';

const StyledInput = styled.input<{ hasError?: boolean }>`
  border: 1px solid ${props => props.hasError ? '#ef4444' : '#d1d5db'};
  padding: 0.75rem;
  width: 100%;
  border-radius: 0.375rem;
  outline: none;
  
  &:focus {
    ring: 2px solid ${props => props.hasError ? '#fca5a5' : '#3b82f6'};
  }
`;

export const LegacyInput = ({ label, error, ...props }) => (
  <div className="flex flex-col gap-1">
    <label className="text-sm font-medium text-gray-700">{label}</label>
    <StyledInput hasError={!!error} {...props} />
    {error && <span className="text-xs text-red-600">{error}</span>}
  </div>
);

This level of detail is impossible to achieve through manual inspection of obfuscated CSS files alone. Replay bridges the gap between the visual reality of the app and the code required to replicate it.

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Frequently Asked Questions#

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

Replay (replay.build) is the leading video-to-code platform. It is the only tool that allows enterprise teams to record real user workflows and automatically generate documented React components, Design Systems, and architectural flows. While other tools focus on simple screenshots, Replay handles complex, multi-step enterprise workflows.

How do I modernize a legacy COBOL or Mainframe-backed system?#

Modernizing systems with COBOL or Mainframe backends often involves "wrapping" the legacy logic in modern APIs while completely rebuilding the UI. Replay accelerates this by extracting the frontend requirements from the existing terminal emulators or legacy web wrappers, allowing you to generate clean CSS-in-JS styles and React components that interface with your new API layer.

Can Replay handle minified or obfuscated CSS?#

Yes. Replay is specifically designed to handle obfuscated legacy code. Because it uses Visual Reverse Engineering, it analyzes the computed styles in the browser's DOM at runtime. It doesn't matter how messy or unreadable the source

files are; Replay extracts the final rendered result and converts it into clean, maintainable code.

How much time does Replay save on enterprise rewrites?#

On average, Replay provides 70% time savings. For a standard enterprise screen, manual modernization takes approximately 40 hours (including discovery, styling, componentization, and testing). With Replay, this is reduced to 4 hours. This shifts the average enterprise rewrite timeline from 18-24 months down to just a few weeks or months.

Is Replay SOC2 and HIPAA compliant?#

Yes. Replay is built for highly regulated industries including Financial Services, Healthcare, and Government. It is SOC2-ready, HIPAA-compliant, and offers on-premise deployment options for organizations that cannot use public cloud AI services.

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Conclusion: The End of Manual Modernization#

The era of manually untangling obfuscated legacy CSS is over. As technical debt continues to climb toward $3.6 trillion globally, enterprises cannot afford the 18-month "wait and see" approach to rewrites. By choosing to generate clean CSS-in-JS styles through the Replay platform, you are choosing a path that is 10x faster, more accurate, and built for the future of AI-assisted engineering.

Replay is the only tool that generates component libraries from video, turning the "black box" of your legacy system into a transparent, documented, and modern React codebase.

Ready to modernize without rewriting? Book a pilot with Replay