Maintain the aspect ratio of a div with CSS

I want to create a responsive div that can change its width/height as the window's width changes. Are there any CSS rules that would allow the height to change according to the width, while maintaining its aspect ratio? I know I can do this via JavaScript, but I would prefer using only CSS.

[tsimo]

アスペクト比を保持するdiv要素の書き方。裏技的な。