Day 14 of 200 Days of Code focused on enhancing CSS skills for building responsive layouts.Key concepts covered include different CSS sizing units like px, em, rem, %, vh, vw, float, clear properties, and Flexbox layout.CSS Sizing Units explained various units like px, em, rem, %, vw, vh in detail for element sizing.Flexbox was introduced as a powerful modern CSS feature simplifying layout design and providing flexibility.