The sources provide an extensive overview of the transition from traditional, fixed-curve web animations to more expressive physics-based (spring) animations that mimic real-world motion. They detail the history and specifications of the proposed but unstandardized spring() timing function in CSS, including its parameters like mass and stiffness, which Apple's WebKit implemented experimentally. Furthermore, the documents explore various current alternatives developers use to achieve spring-like effects, such as approximations using linear() and keyframe hacks in CSS or utilizing high-performance, off-main-thread JavaScript libraries and Animation Worklets (part of Houdini). Finally, the text explains the complex implementation challenges within modern browser engines like Blink/Chromium, highlighting the critical need to integrate the physics calculations into the Compositor Thread (cc) for smooth, jank-free performance.