Modern CSS & HTML

Write CSS and HTML with current platform features. Prefer native materials over legacy hacks and unnecessary JavaScript.

Composition with other skills

• This skill owns native implementation choices.
• fluid-design owns interaction feel when physics, interruption, or gestures exceed CSS.
• semantic-html-first owns element and ARIA boundaries.
• If a native CSS/HTML feature can meet the requirement with acceptable support, use it.

Browser support policy

Target features available in both WebKit (Safari) and Blink (Chrome/Edge) unless the task explicitly allows a single-engine experiment.

• Widely available in both engines: ship as default.
• Newly available in both: ship with @supports / progressive enhancement.
• Single-engine or unstable: mention as optional; provide a cross-engine path.

Do not hardcode volatile global percentage figures in recommendations. Re-check support when unsure, and encode the decision as "both engines / enhance / fallback".

Core defaults

HTML

• Use native elements (button, a, details, dialog, select, form controls) before ARIA widgets.
• Prefer semantic structure (main, nav, section, headings) over div soup.
• Use Popover API + CSS Anchor Positioning before custom floating stacks when support allows.

Layout

• gap over margin hacks; inset over four physical offsets; aspect-ratio over padding hacks.
• 100dvh for mobile full-height; logical properties for direction-aware spacing.
• Container queries for component responsiveness; range syntax for media queries.
• subgrid, grid-template-areas, position: sticky, scroll snapping when they replace JS.

Colour and theme

• Build palettes in oklch().
• Derive states with color-mix() and relative colour syntax.
• Prefer light-dark() with color-scheme over duplicated scheme blocks.

Motion

• Animate translate, rotate, scale, and opacity preferentially.
• Use @starting-style and transition-behavior: allow-discrete for entry/exit where supported.
• Prefer CSS scroll-driven timelines over Intersection Observer class toggles when support allows.
• Defer to fluid-design for springs, gestures, and interruptible physics.

Review checklist

• Is there a native HTML element for this control?
• Can layout/colour/motion be done in CSS without a library?
• Are colours tokens/oklch rather than one-off hex?
• Are fallbacks expressed with @supports where needed?
• Does reduced-motion get a real alternative path?
• Are physical left/right properties avoided for direction-sensitive UI?
• If JS remains, is it covering a gap CSS/HTML cannot?

References

• Read [Task recipes](references/recipes.md) when implementing a matching pattern.
• Read [Feature notes](references/features.md) when choosing a feature or fallback.

Load only the reference relevant to the current decision.