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| Concern | What to inspect | Typical failure |
|---|---|---|
| Core behavior | Keyframe Animations | Animations are not automatically reversible when state changes. If the interaction is a simple state transition, a transition can often express the intent more clearly. |
| Production test | Real content and constraints | The rule works only for the demo values |
| Maintenance | Scope and override path | A later fix creates another exception |
In production, Animations are not automatically reversible when state changes. If the interaction is a simple state transition, a transition can often express the intent more clearly. That is the kind of bug that survives visual spot checks because the stylesheet is syntactically valid while the layout contract is wrong.
<style>@keyframes fade-in { from { opacity:0; } to { opacity:1; } }</style><main class="demo">Example content</main>
The useful mental model here is practical: how @keyframes defines multi-step animated state and how animation properties bind those steps to an element. When a rule looks obvious but the browser produces something else, start by identifying which constraint or rendering phase is actually winning.
A good CSS change narrows the problem. Use keyframes when the motion has a timeline rather than a single start and end state. This keeps the rule understandable when content, theme, or viewport conditions change later.
<style>.toast { animation:fade-in .3s ease-out both; }</style><main class="toast">Example content</main>
Engineering judgment: Use keyframes when the motion has a timeline rather than a single start and end state.
<style>@keyframes pulse { 0%,100% { transform:scale(1); } 50% { transform:scale(1.02); } }</style><main class="demo">Example content</main>
Two-step motion is useful here because it demonstrates keyframe animations under a concrete input rather than as an abstract property list.
<style>.demo { animation-fill-mode:both; }</style><main class="demo">Example content</main>
Explicit stop is useful here because it demonstrates keyframe animations under a concrete input rather than as an abstract property list.
Finish the lesson once you have worked through the material. This awards ★ 30 XP.