Create a fully functional, single-file HTML countdown timer application with the following exact requirements:

Core Timer Functionality:

• Large timer display: 4rem font size, MM:SS format, centered, using monospace font

• Preset time buttons: 5, 15, 25, 30, 45, 60 minutes (clearly labeled)

• Custom time input: Number input field with "Set" button, 1-999 minute range

• Start/Stop control: Single button that toggles between "Start" and "Stop"

• Manual reset: Separate "Reset" button that restores original time

• Add time while running: +1min, +5min, +10min buttons (only visible when timer is active)

Timer completion alerts:

• Visual: Brief yellow background flash animation

• Audio: 0.5-second beep sound using Web Audio API Modal/popup message: "Timer Complete!" for 3 seconds

Task Management System:

• Add task form: Two inputs (task name, duration in minutes) + "Add Task" button

• Task list display: Shows all tasks with name, duration, and action buttons

• Start from task: Button to start timer with task's duration and name

• Mark complete: Button to mark task as done (shows strikethrough styling)

• Completed tasks: Visual indication with strikethrough and reduced opacity

• Data persistence: Must use localStorage to save tasks between sessions

History & Progress Tracking:

• Completion tracking: Automatically log completed timer sessions

• History display: Last 10 completed sessions with:

  • Task/session name

  • Duration

  • Completion date and time (formatted: MM/DD/YYYY at HH:MM AM/PM)

• Persistent storage: Use localStorage for history data

Design Requirements:

• Color scheme: White background, black text, blue buttons (#007bff)

• Typography: System fonts (-apple-system, BlinkMacSystemFont, 'Segoe UI', etc.)

• Layout: Clean sections with light gray borders and backgrounds (#fafafa) Responsive design: Works on mobile (320px+) and desktop

• Button styling: Consistent padding, hover effects, disabled states

Technical Implementation Requirements:

• Single HTML file: All CSS and JavaScript embedded No external dependencies: No CDN links, libraries, or frameworks

• Error handling: Try-catch blocks for localStorage operations Input validation: Check for empty/invalid values with user feedback Keyboard support: Enter key works in all input fields Accessibility: Proper semantic HTML, labels, and ARIA attributes

Critical Success Factors:

• Use direct button clicks, not form submissions for task management Include comprehensive error handling with user-friendly messages

• Test all localStorage operations with try-catch blocks

• Validate all user inputs before processing

• Provide visual feedback for all user actions Ensure mobile responsiveness with proper viewport and media queries

Layout Structure (Top to Bottom):

• App Title: Large, centered heading Timer

• Section: Display + controls + preset buttons + custom input

• Task Management: Add form + task list

• History Section: Completed sessions list

Specific Code Requirements:

• Use localStorage.setItem() and localStorage.getItem() for data persistence Include JSON.stringify() and JSON.parse() for object storage Use setInterval() and clearInterval() for timer functionality

• Implement Web Audio API beep with oscillator Use CSS flexbox for responsive layouts Include proper error messages and validation

Testing Checklist

• The final application must:

  • Timer starts, stops, and resets correctly

  • Preset buttons set timer to correct values

  • Custom time input accepts and applies values

  • Add time buttons work while timer is running

  • Tasks can be added with name and duration

  • Tasks persist after page reload

  • Timer can be started from task with correct duration

  • Tasks can be marked as complete

  • History shows completed sessions

  • All buttons are clickable and responsive

  • Works on mobile devices

  • Audio beep plays on completion

  • Visual flash occurs on completion

Note: If localStorage doesn't work in the environment, use in-memory storage (JavaScript variables/arrays) and mention this limitation to the user. This specification ensures a production-ready timer application that will work reliably for focused work sessions and productivity tracking.