Interface motion is 100-300ms with standard easing; no decorative motion on critical paths

Requirement

Interface motion runs between 100ms and 300ms with standard easing. Below 100ms a transition reads as a jump; above 300ms it reads as a wait. Standard easing means an ease-out or ease-in-out curve — never bounce or elastic, which SLP-8 forbids outright.

On a critical path — the steps a user takes to complete a task — motion is limited to functional feedback: a state change, a control responding, a surface arriving. Decorative motion belongs elsewhere.

transition-all is always a finding. It animates every property that happens to change, including layout properties that cannot be animated cheaply, and it makes the set of animated properties impossible to reason about.

Rationale

Motion on a task flow is a cost paid by every user on every repetition. A 500ms reveal is pleasant once and an obstacle the fiftieth time, and the people using these surfaces are using them all day. The 100-300ms band is where a transition is fast enough to feel like a response and slow enough to be followed.

The boundary with the neighbouring motion controls: MOT-1 bounds where motion may run and how long; MOT-2 bounds where its values come from; MOT-3 bounds what it may carry. A11Y-5 requires a reduced-motion variant to exist, which is a separate obligation from any of the three.

Why this is hybrid

Static-check half. Durations, easing functions, and transition-all are literal values in stylesheets and component source, and finding those that fall outside the band is mechanical. That half is planned in #157 as checks/motion-scan.py; no script exists yet.

Judgment half. "Critical path" and "decorative" are not properties of the source. A 300ms transition is legal by the band and still wrong if it sits between two steps of a form a user completes forty times a day, and a longer duration can be right on a first-run surface nobody repeats. Deciding which flows are critical, and which motion is functional feedback rather than decoration, is the evaluator's.

Passes when

• Interface transitions land between 100ms and 300ms.
• Easing is a standard ease-out or ease-in-out curve.
• Motion on task flows is limited to functional feedback.
• Transitions name the properties they animate.

Fails when

• A transition on a task flow runs longer than 300ms.
• transition-all appears anywhere in product UI.
• Decorative motion — a reveal, a parallax, an ambient loop — runs on a critical path.

Evaluator guidance

Flag:

• Durations outside the 100-300ms band on interface elements.
• transition-all, without exception.
• Decorative motion on a surface a user passes through repeatedly to finish a task.

Do not flag:

• Narrative or marketing surfaces that are not task flows, where longer motion is a deliberate choice with a recorded rationale.
• Loading and progress indicators, whose duration is set by the work rather than by the designer.
• Motion whose value comes from the declared motion token set — MOT-2 governs that question, and a token-sourced duration still has to sit inside this band.
• A recorded rationale for a duration outside the band — this is an L2 control with waiver: rationale.