WhatTheCSS is a Chrome extension for designers and front-end developers that lets you hover over any element on a website to instantly see its authored CSS. Key features include: - Hover-to-inspect: View real declarations, pseudo-classes, and variables as you move your cursor. - Peek mode: Toggle with V to see type, colors, and WCAG contrast. - Live editing: Press Space to freeze an element into a four-tab editor (CSS, Contrast, Box, and Tree) where you can test changes live.