In this blog post, the author demonstrates how to build a realtime browser-based closed-captioning system using React, AssemblyAI's realtime transcription service, and Ably. The application consists of two modes - publisher and subscriber. The publisher captures audio from the user's microphone, transcribes it in real time using AssemblyAI, and broadcasts the transcriptions to subscribers via Ably. The subscriber receives these transcriptions and displays them on the screen. This system can be useful for making content more accessible to people with hearing impairments or those learning a new language.