[Good First Issue] Implement CSS transition animation playback

[wangnaihe]

Module

w3cos-runtime/src/window.rs

Description

The transition data structure and easing functions already exist in w3cos-std/src/style.rs (Transition, Easing). Implement the frame loop in the window event loop that interpolates style values over time when they change.

Acceptance Criteria

• When a style property changes and has a transition defined, animate the change over the specified duration
• Support easing functions: Linear, EaseIn, EaseOut, EaseInOut
• Smooth 60fps animation using the existing winit event loop

Difficulty

Medium

Relevant Files

• crates/w3cos-std/src/style.rs — Transition and Easing types
• crates/w3cos-runtime/src/window.rs — Event loop where animation should happen

[github-actions]

AI Triage — Auto-classified this issue.

Labels: module:runtime, module:dom, difficulty:low, good first issue, ai-ready

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[wangnaihe]

Implemented in Phase 1 (commit da056e6). CSS transition animation with 60fps frame loop, opacity and background interpolation, easing support.