A Keras.js developer creates and implements machine learning models in the web browser using the Keras.js library, which allows models to be run in JavaScript. They design, train, test, and tweak models using Keras, a high-level neural networks API, and then convert these models to run on Keras.js. They utilize web technologies to visualize and interact with the models in real-time. The developer also optimizes the models for speed and performance in the browser. They may work on various applications like image recognition, natural language processing, and more, while ensuring the efficient execution of machine learning models.