CSS

Predictability is the highest CSS virtue. If your styles require !important to work, restructure the cascade.

CSS rewards explicit, low-specificity selectors and intentional cascade ordering. Prefer boring, readable patterns over clever one-liners.

References

Topic	Reference	Contents
Layout	references/layout.md	Flex shorthand values, grid details (subgrid, implicit rows, alignment), layout patterns
Modern CSS	references/modern-css.md	Extended modern CSS patterns and examples
SCSS	references/scss.md	@forward patterns, module configuration, built-in modules, file organization
Responsive	references/responsive.md	Extended responsive design patterns and examples
Methodologies	references/methodologies.md	Methodology patterns and architecture details

Selectors and Specificity

• Single class selectors by default — keep specificity flat at 0-1-0
• Never use ID selectors for styling — IDs are for anchors and JS hooks
• Never qualify classes with elements — .error not div.error
• Max nesting depth: 3 levels — deeper nesting couples CSS to DOM structure
• Avoid !important — use cascade layers or restructure selectors instead; only valid use is in a low-priority reset layer for truly essential styles
• Use :where() to zero-out specificity when needed — :where(.card) .title has 0-0-1 specificity
• Use :is() with awareness — it takes the highest specificity of its arguments
• Flatten nested selectors in SCSS — ability to nest does not mean you should

Layout Systems

Choosing Flexbox vs Grid

Use Case	System
One-dimensional flow (row or column)	Flexbox
Two-dimensional layout (rows AND columns)	Grid
Content-driven sizing	Flexbox
Layout-driven sizing	Grid
Component internals (nav items, card content)	Flexbox
Page-level structure, complex arrangements	Grid
Items need to wrap naturally	Flexbox
Precise placement on named lines/areas	Grid

Both work together — a grid item can be a flex container and vice versa.

Flexbox

• Always use the flex shorthand — it sets intelligent defaults. See references/layout.md for the full shorthand value table
• flex-flow: row wrap combines flex-direction and flex-wrap
• Use flex-wrap with a flex basis for responsive layouts without media queries: flex: 1 1 300px wraps items when they can't maintain 300px minimum
• Centering: display: flex; align-items: center; justify-content: center or margin: auto on a flex child
• gap over margin hacks — works in both flexbox and grid
• Avoid justify-content: space-between with wrap — causes orphan gaps; prefer gap + flex-wrap

CSS Grid

• repeat(auto-fit, minmax(250px, 1fr)) is the canonical responsive grid — no media queries needed
• Prefer auto-fit over auto-fill — auto-fit expands columns to fill space; auto-fill keeps empty tracks
• Use named grid areas for page-level layouts — they auto-create named lines
• Never hardcode px widths on grid items — use fr, minmax(), or auto
• grid-auto-flow: dense fills visual holes — use carefully, it breaks visual/source order alignment (a11y concern)
• Never use order in ways that break logical reading order
• See references/layout.md for subgrid, implicit rows, alignment shorthands, and negative line numbers

General Layout Rules

• Never use float for layout — floats are for wrapping text around images
• Intrinsic sizing first — use flex-wrap, min(), max(), clamp() before reaching for media queries
• gap over margin hacks in both flexbox and grid

CSS Nesting

• Use & for pseudo-classes/elements and compound selectors — &:hover, &::before, &.active
• Omit & for descendant selectors — .card { .title {} } works
• & is required when the nested selector starts with a type selector — & p {} not p {}
• Nesting at-rules (@media, @supports, @container) nest directly inside rules
• Specificity: :is() wrapping applies in nesting — be aware that specificity may differ from the equivalent unnested selector
• Max depth: 3 levels — same rule as flat CSS

Cascade Layers (@layer)

• Declare all layers at the top of the stylesheet in a single statement: @layer reset, defaults, themes, components, utilities;
• First declared = lowest priority; un-layered styles always beat layered styles
• !important reverses layer order — !important in the lowest layer wins over !important in higher layers
• Import third-party CSS into sub-layers: @import url('vendor.css') layer(vendor.bootstrap);
• Use revert-layer to roll back to the previous layer's value
• !important in low layers is intentional — it means "this style is essential, don't override"
• Don't create layers per-component — layers manage cascade priority between categories (reset vs component vs utility), not scope
• Nested layers: @layer components { @layer buttons, cards; } — access via @layer components.buttons
• Anonymous layers (@layer { }) can't be appended to later

Container Queries

• Define containment: container-type: inline-size on the wrapper
• Name containers for targeting: container: card / inline-size
• Query by name: @container card (width > 400px) { }
• Unnamed queries hit the nearest ancestor container

Container Query Units

Unit	Meaning
cqw / cqh	1% of container width / height
cqi / cqb	1% of container inline / block size
cqmin / cqmax	Smaller / larger of cqi or cqb

Use cqi instead of vw for container-scoped fluid values: font-size: clamp(1rem, 2.5cqi + 0.5rem, 2rem)

Responsive Design

Responsive Hierarchy

Design from the inside out — use the right tool for each level:

Level	Tool	When
Content-driven	Flexbox wrapping, min()/max()/clamp()	Always — baseline
Container-driven	Container queries, cqi/cqw units	Component adapts to parent
Viewport-driven	Media queries, vw/vh/dvh	Page-level layout changes
User preference	prefers-* media queries	Color scheme, motion, contrast

Core Rules

• Mobile-first — default styles for small screens, enhance upward
• Content-driven breakpoints — let content decide, not device sizes
• rem for breakpoints: @media (width >= 45rem) not (min-width: 768px)
• Use modern range syntax: @media (768px <= width < 1024px)
• Logical properties for layout: margin-inline-start not margin-left
• Container queries for component-level adaptation; media queries only for viewport-dependent elements (nav, header)
• Respect user preferences: prefers-reduced-motion, prefers-color-scheme, prefers-contrast
• Single container max-width pattern: width: min(100% - 2rem, 75rem); margin-inline: auto — avoid multiple max-width values at different breakpoints

Fluid Sizing

• clamp(min, preferred, max) for fonts, spacing, and container widths
• Build a fluid type scale with custom properties: --step-0: clamp(1rem, 0.5rem + 1.5vw, 1.25rem)
• Never use vw alone for font size — it blows up on large screens; always pair with clamp() and rem
• Use cqi instead of vw for container-scoped fluid values

Logical Properties

Use logical properties for layout-sensitive values (margins, padding, borders, text alignment, positioning offsets). Physical properties are fine for visual effects not affected by writing direction (e.g., box-shadow offsets).

Physical	Logical
left / right	inline-start / inline-end
top / bottom	block-start / block-end
width / height	inline-size / block-size
margin-left	margin-inline-start
padding-top	padding-block-start
text-align: left	text-align: start

Shorthands: margin-block: 1rem 2rem (block-start, block-end); margin-inline: auto (both inline directions).

User Preference Queries

• Color scheme: declare color-scheme: light dark on :root and use @media (prefers-color-scheme: dark) for overrides
• Reduced motion: either remove motion in prefers-reduced-motion: reduce or add motion only in prefers-reduced-motion: no-preference (progressive enhancement approach)
• High contrast: @media (prefers-contrast: more)
• Interaction: @media (hover: hover) for hover effects on capable devices; @media (pointer: coarse) for touch targets (min 44px)

Responsive Images

• img { max-width: 100%; height: auto; display: block; }
• Use aspect-ratio + object-fit: cover for hero images

The :has() Selector

Select elements based on descendants or siblings — the "parent selector."

• Anchor to specific elements, not body, :root, or * — broad anchors force expensive re-evaluation on every DOM change
• Use direct child (>) or sibling (+, ~) combinators inside :has() to limit traversal scope
• Cannot nest :has() inside :has()
• Pseudo-elements are not valid inside :has()
• .layout:has(> .sidebar-open) (good) not body:has(.sidebar-open) (bad)

Custom Properties

• Define design tokens on :root — scope overrides to components
• Semantic naming: --color-text-primary not --dark-gray
• Use kebab-case; prefix with category: --color-, --spacing-, --font-
• Provide fallbacks for component-level variables: var(--button-bg, var(--color-primary))
• Custom properties are case-sensitive — --my-color differs from --My-Color
• Custom properties inherit by default (unlike most CSS properties)
• Use @property for typed, animatable custom properties — enables type checking (invalid values fall back to initial-value), controlled inheritance (inherits: false), and transitions on custom properties

View Transitions

• view-transition-name must be unique per page at transition time
• Keep transitions short — 200-400ms for UI, longer for page-level
• Always respect prefers-reduced-motion: reduce for view transitions
• Same-document (SPA): document.startViewTransition(() => { /* update DOM */ })
• Cross-document (MPA): @view-transition { navigation: auto; }
• Named transitions target specific elements via ::view-transition-group(name)

Box Model and Sizing

• Always set box-sizing: border-box globally via reset
• rem for font sizes and breakpoints — respects user preferences
• em for component-relative spacing (padding that scales with font size)
• Fluid sizing with clamp() — replace manual breakpoint ladders
• aspect-ratio over padding hacks for maintaining proportions
• No units on zero values — margin: 0 not margin: 0px (except where required: flex: 0 0 0px)
• Leading zero on decimals: opacity: 0.5 not opacity: .5
• Shorthand hex where possible: #ebc not #eebbcc

SCSS / Dart Sass

Module System

• @use and @forward only — @import is deprecated (Dart Sass 1.80.0), removed in 3.0.0
• @use must appear before any rules except @forward
• Namespace defaults to the last component of the URL (without extension)
• Members are scoped to the loading file — not globally available
• Each module loaded exactly once — no duplicate CSS output
• Namespace access: variables.$primary not global $primary
• No-namespace @use 'variables' as * — use sparingly, only for own files
• math.div() for division — the / operator is deprecated
• Prefix private members with - or _
• Prefer mixins over @extend — more predictable output; @extend produces unexpected selectors and doesn't work across media queries
• Max nesting: 3 levels

@forward re-exports modules, supports prefixing and visibility control. Module configuration uses !default variables and @use ... with (). See references/scss.md for @forward patterns, configuration passthrough, built-in module usage, and file organization conventions.

Migration

Use the official migrator: sass-migrator module --migrate-deps entrypoint.scss. For built-in functions only: sass-migrator module --built-in-only entrypoint.scss.

CSS Methodologies

BEM (Block Element Modifier)

• Blocks are standalone components: .card, .nav, .form
• Elements are parts of a block (double underscore): .card__title, .card__image
• Modifiers are variations (double hyphen): .card--featured, .card__title--bold
• Never nest elements: .card__header__title is wrong — flatten to .card__title or create a new block
• Modifiers don't exist alone — always pair with base class: class="card card--featured"
• Use BEM in team projects, large codebases, projects without scoped styles
• Skip BEM when using CSS Modules, utility-first CSS, or small projects

CSS Modules

• Use simple, descriptive class names — scoping eliminates conflict risk
• One module per component
• Compose shared styles: composes: resetButton from './shared.module.css'
• Global escape hatch: :global(.utility-class) when needed
• Pair with custom properties for theming (variables aren't scoped)

Architecture (ITCSS)

Organize styles by specificity, low to high: Settings → Tools → Generic → Elements → Objects → Components → Utilities. Maps naturally to cascade layers: @layer settings, generic, elements, objects, components, utilities;

Formatting

• 2-space indentation, no tabs
• One declaration per line
• Semicolon after every declaration including the last
• Space after colon: color: red not color:red
• Opening brace on same line as selector
• Blank line between rules
• Lowercase everything (selectors, properties, values, hex colors)
• Single quotes for attribute selectors and font names
• Group declarations by category: layout → box model → typography → visual → interaction

Application

When writing CSS:

• Apply all conventions silently — don't narrate each rule being followed.
• Use intrinsic sizing and fluid techniques before media queries.
• If an existing codebase contradicts a convention, follow the codebase and flag the divergence once.

When reviewing CSS:

• Cite the specific violation and show the fix inline.
• Don't lecture or quote the rule — state what's wrong and how to fix it.

Bad review comment:
  "According to CSS best practices, you should avoid using ID selectors
   for styling because they have high specificity."

Good review comment:
  "`#header` -> `.header` -- IDs create specificity 1-0-0, difficult to override."

Integration

The coding skill governs workflow; this skill governs CSS implementation choices. For SCSS, this single skill covers both CSS and SCSS conventions.

Predictability is the highest CSS virtue. When in doubt, keep specificity low and cascade explicit.