elevation-and-depth（高程和深度）

Elevation and Depth

Elevation uses shadow and layering to communicate that an element sits above the base surface — giving UI a sense of physical depth. Combined with border-radius, it creates the tactile quality that makes cards graspable, modals clearly floating, and dropdowns feel like they've appeared on top of content.

The Elevation Scale

Define a small set of elevation levels as tokens. Each level maps to a specific UI role.

Keep shadows subtle. Dark, heavy shadows feel dated and visually aggressive. Light, diffuse shadows feel modern and material.

The "Shadow + Border" Rule

A subtle shadow on a white surface can sometimes "wash out," making the edge of a card feel fuzzy or indistinct.

• Rule: For elevated white cards or sections, pair the shadow with a 1px border that is slightly darker (1.5x to 2x) than the shadow's core tone (e.g., a medium grey like #E2E8F0 or grey-200).
• Effect: The border defines the physical boundary of the card, while the shadow provides the depth. Together, they make the component "pop" with much higher clarity than using either alone.

Subtle Gradients for Depth

Gradients can be used to bring "liveliness" to an interface and reinforce the sense of elevation.

• The Lighting Metaphor: A subtle linear gradient (top-to-bottom) that is slightly lighter at the top mimics natural overhead lighting. This makes a surface feel more physical and elevated than a flat fill.
• The 5% Rule: Keep the gradient extremely subtle. A change in lightness of only 2–5% between the top and bottom is usually enough. If the user can easily see where the gradient starts and ends, it is likely too heavy.
• Usage: Apply to primary buttons, hero cards, and header sections to improve visual hierarchy and brand personality.

Pairing Elevation with Border-Radius

Border-radius and shadow work together to define the character of a surface. The combination signals the element's role and the product's visual tone.

Border-Radius Consistency Rule

Button border-radius must not vary within a product. All buttons — primary, secondary, destructive, ghost — use the same radius token. Varying radius between button types breaks visual consistency and implies a semantic difference that does not exist.

/* Correct: one radius for all buttons */
.btn { border-radius: var(--radius-button); }

/* Wrong: different radii for different button variants */
.btn-primary { border-radius: 8px; }
.btn-secondary { border-radius: 4px; }  /* ← breaks consistency */

The button radius token is a brand decision — set it once, apply it everywhere.

Text Shadow for Contrast and Separation

text-shadow can lift text off a background without changing colours — useful when contrast is marginal or text sits on a photograph, gradient, or complex background.

The effect must be imperceptible as a shadow. If the user notices the shadow, it is too strong.

/* On images or complex backgrounds */
.hero-title {
  text-shadow: 0 1px 3px rgba(0, 0, 0, 0.25);
}

/* Very subtle separation from a near-matching background */
.label-on-tinted-surface {
  text-shadow: 0 1px 2px rgba(0, 0, 0, 0.12);
}

/* White text on light background — use a dark shadow, not white */
.inverted-text {
  text-shadow: 0 1px 4px rgba(0, 0, 0, 0.30);
}

Rules:

• Opacity: stay below 0.30 — above that it reads as a design choice rather than a refinement
• Blur: 2–4px maximum — higher values make text feel dirty
• Offset: 0 1px or 0 0 — directional offsets above 2px look like retro Web 2.0
• Never use text-shadow to compensate for a contrast failure — fix the colour first. Use it only when colours are already close to passing and a nudge is needed, or when text sits on unpredictable imagery

Dark Mode Shadows

Shadows are less visible on dark backgrounds. On dark surfaces, compensate with:

• Slightly higher opacity on shadow values
• Adding a subtle border (1px solid rgba(255,255,255,0.08)) to define card edges
• Increasing the elevation level by one step for the same perceived depth

Review Checklist

• Is there a defined elevation scale as design tokens (not one-off shadow values per component)?
• Does each elevated element use the correct level for its role (card ≠ modal)?
• Are shadows light and diffuse, avoiding heavy black (#000) defaults?
• For white cards on light backgrounds, is the shadow paired with a 1px border for better definition?
• Is border-radius consistent across all button variants?
• Do shadows feel subtle and diffuse rather than heavy and dark?
• On dark surfaces, are card/panel edges visible through border or adjusted shadow?
• Is elevation used to communicate layering — not just decoration?
• If gradients are used, are they subtle (2–5% lightness change) and used to reinforce the lighting/elevation metaphor?

Common Anti-Patterns