<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <title>CSS Preventing Change in Element Box Dimensions</title> <style> div { width: 100%; padding: 20px; box-sizing: border-box; background: green; } </style> </head> <body> <div> <h1>Happy Morning..</h1> </div> <p><strong>Notice</strong> Enjoy your day..</p> </body> </html>