GitHub emphasizes the importance of JavaScript performance and browser compatibility, adopting strategies like updating compilers to support modern ECMAScript standards, such as ES2019 and ECMAScript 2020, to enhance user experience. This approach leverages progressive enhancement, ensuring users on older browsers can still access basic features, while those on modern ones enjoy improved performance. GitHub uses analytics, such as parsing User-Agent headers, to understand browser usage patterns, which inform decisions on browser support and feature deployment. Chrome dominates with over 70% of traffic, followed by Firefox, Edge, and Safari. To maintain code compatibility across diverse browsers, GitHub employs linting and polyfills, though they focus on a minimal set of polyfills to optimize performance. Tools like the browser-support library help determine feature support and manage compatibility issues effectively. GitHub's commitment to delivering a fast and reliable service is supported by data-driven decisions and tools that align with their principles, ensuring seamless interactions for users across different browser environments.