Technical Specification: Free Online Percentage Change Calculator

Overview

A high-performance, single-file browser utility designed to perform instant percentage calculations. This tool is built to provide financial analysts, students, and e-commerce managers with a fast, reliable, and accessible interface for determining data variance.

Core Functionality

• Percentage Change Mode: Calculate the difference between an initial value and a final value as a percentage.
• Percent Increase Mode: Calculate the final value after applying a specific percentage increase.
• Percent Decrease Mode: Calculate the final value after applying a specific percentage decrease.
• Real-time Computation: Results appear instantly as users type, using vanilla JavaScript for zero-latency feedback.
• Precision Controls: Adjustable decimal rounding (up to 4 decimal places).
• In-Memory History: A session-based results log that allows users to track their current calculations without relying on persistent storage (reset on page refresh).

UI/UX Design

• Layout: A clean, card-based interface centered on the screen. The input area uses large, high-contrast typography.
• Color Palette: A vibrant, professional light-mode aesthetic.
  • Primary: #2563eb (Royal Blue) for primary actions and results.
  • Background: #f8fafc (Slate Gray) for the page background.
  • Surface: #ffffff (White) for the calculator container with a subtle soft shadow (shadow-lg).
  • Text: #1e293b (Slate-800) for high readability.
• Animations: Subtle CSS-driven fade-in effects for results and smooth hover states for buttons using CSS transitions (transition-all duration-300).

Technical Constraints & Requirements

• Single File: All HTML, CSS (using a CDN link for Tailwind CSS), and JS must reside in one .html file.
• No Persistence: Do NOT use localStorage, sessionStorage, or cookies. The application must operate purely in-memory. If a user refreshes the page, the history should clear.
• Iframe Compatibility: The app must be fully functional within a null-origin iframe (no popups, no external dependencies that require top-level window access).
• Accessibility: All inputs must have associated <label> tags and ARIA labels. The tool must be navigable via keyboard (Tab index management).
• Performance: Zero external dependencies other than Tailwind CSS and Google Fonts via CDN. No build step required.