Create a single-page SPA (Single Page Application) using HTML, CSS, and JavaScript. The application should allow users to interact with a hypothetical 'GPT-5.3' AI model. The core functionality is to send a text prompt from the user to the AI and receive an instant text response.
**Core Features:**
1. **Input Area:** A textarea for the user to type their prompt.
2. **Send Button:** A button to submit the prompt.
3. **Response Area:** A display area to show the AI's response. This area should be dynamically updated.
4. **Loading Indicator:** Show a simple loading indicator (e.g., 'Thinking...') while waiting for the AI response.
5. **Basic Styling:** Apply clean and modern CSS for a user-friendly interface. Use a responsive design that works on desktop and mobile.
**Technical Requirements:**
* No external frameworks (like React, Vue, Angular) or libraries (like jQuery). Pure HTML, CSS, and vanilla JavaScript.
* Simulate the AI response: Since we don't have a real GPT-5.3 API, use `setTimeout` to simulate a network delay (e.g., 1-2 seconds) and then display a placeholder response like "This is an instant response from GPT-5.3 for your query: '[User's Query]'. This is a simulated response."
* The SPA structure should be well-organized with clear HTML, CSS, and JavaScript files (or sections within a single HTML file if preferred, but clearly separated).
* Ensure the JavaScript is efficient and handles UI updates correctly.
* The `claudeMasterPrompt` variable itself should not be included in the generated code, but rather the instructions for generating the SPA.