Write a function `rectangle_area(width, height)` that returns the area of a rectangle. The area is calculated as `width * height`. The inputs will always be non-negative integers or floats. The function must return the exact product as a number (int or float).
Constraints
- 0 <= width, height <= 10^9
- Inputs are int or float.
- The result may be a float if either input is a float.