Act as a senior JavaScript developer with 10+ years of experience in building interactive web applications. Design a step-by-step guide for creating a fully functional calculator using [vanilla JavaScript], [HTML], and [CSS]. Ensure the calculator includes basic operations like [addition, subtraction, multiplication, and division], as well as a [clear button] and support for [decimal points]. Provide detailed explanations for each step, including how to structure the HTML layout, style the calculator with CSS, and implement the logic in JavaScript. Highlight best practices for clean code, such as modular functions, error handling, and efficient DOM manipulation. Additionally, suggest how to test the calculator for usability and edge cases, such as dividing by zero or handling large numbers.