Google developed Native Client (NaCl) with the intent to safely run native code within a web browser. The executable code would run in a sandbox and offered the performance advantages of native code execution.

NaCl was tied to a specific architecture, while Portable Native Client (PNaCl) was an architecture-independent version of NaCl developed to run on any platform. The technology consisted of two elements:

• Toolchains which could transform C/C++ code to NaCl modules
• Runtime components which were components embedded in the browser that allowed execution of NaCl modules:

NaCl's architecture-specific executable (nexe) was limited to applications and extensions that were installed from Google's Chrome Web Store, but PNaCl executables (pexe) can be freely distributed on the web and embedded in web applications. Portability was made possible with Pepper, an open source API for creating NaCl modules, and its corresponding plugin API (PPAPI). Pepper enabled communication between NaCl modules and the hosting browser, and allowed for access to system-level functions in a safe and portable way. Applications could be easily distributed by including a manifest file and a compiled module (pexe) with the corresponding HTML, CSS, and JavaScript:

NaCl offered promising opportunities to overcome the performance limitations of the web, but it had some drawbacks. Although Chrome had built-in support for PNaCl executables and Pepper, other major browser did not. Detractors of the technology took issue with the black-box nature of the applications as well as the potential security risks and complexity.

Mozilla focused its efforts on improving the performance of JavaScript with asm.js. They wouldn't add support for Pepper to Firefox due to the incompleteness of its API specification and limited documentation. In the end, NaCl was deprecated in May, 2017, in favor of WebAssembly.