Issue 04 | Phase 0: Project Initialization and Core Requirements Analysis

4.1 Phase 0: Project Initialization and Planning

4.1.1 Creating the Requirements Document

requirement.md — The authoritative source for project requirements:

# Web Calculator - Requirements Document

## Overview
A single-page web calculator, offering two UI modes: Senior Mode and Standard Mode.
Pure HTML/CSS/JS implementation, no frameworks, no backend.

## Core Features
- Basic operations: + - × ÷ %
- Parentheses support, sign toggle
- History: last 10 entries
- Full keyboard support
- WCAG AAA accessibility
- Responsive: Desktop/Tablet/Mobile

## Senior Mode
- Buttons 80×80px, font size 28px/36px
- High contrast WCAG AAA
- Hide advanced operations (parentheses, %)
- Long press backspace to clear all

## Standard Mode
- Buttons 48×48px, font size 18-20px
- Modern minimalist color scheme
- Full functionality

## Technical Constraints
- Only three files: index.html, style.css, app.js
- No external dependencies
- Offline support