Modern web browsers have effectively become the new operating system for business applications, yet their security posture is often under-scrutinized. This paper presents a novel, comprehensive Browser Security Posture Analysis Framework[1], a browser-based client-side security assessment toolkit that runs entirely in JavaScript and WebAssembly within the browser. It performs a battery of over 120 in-browser security tests in situ, providing fine-grained diagnostics of security policies and features that network-level or os-level tools cannot observe. This yields insights into how well a browser enforces critical client-side security invariants. We detail the motivation for such a framework, describe its architecture and implementation, and dive into the technical design of numerous test modules (covering the same-origin policy, cross-origin resource sharing, content security policy, sandboxing, XSS protection, extension interference via WeakRefs, permissions audits, garbage collection behavior, cryptographic APIs, SSL certificate validation, advanced web platform security features like SharedArrayBuffer, Content filtering controls ,and internal network accessibility). We then present an experimental evaluation across different browsers and enterprise scenarios, highlighting gaps in legacy browsers and common misconfigurations. Finally, we discuss the security and privacy implications of our findings, compare with related work in browser security and enterprise endpoint solutions, and outline future enhancements such as real-time posture monitoring and SIEM integration.