Platform & CSS

A lab for the modern web platform. Each section is a live, interactive demo of a CSS or HTML feature that used to need JavaScript, a media-query soup, or a brittle override — and now ships in the browser. Every example is WCAG 2.2 AAA, token-driven where it earns its keep, and SSR-safe. The short note on each one says what it replaces and where it touches accessibility or internationalisation.

Container queries

Components shouldn't care how wide the viewport is — they should respond to the space they are given. @container lets a card restyle off its container's width, so the same component is correct in a sidebar, a wide main column, or a modal without per-page media queries. Drag the slider and watch the card flip its layout at its own breakpoint — the viewport never changes.

Container width: 60%

Responsive to its container

Below the container's breakpoint this stacks; above it, media and text sit side by side. Resize the box, not the window.

Current layout: inline

Accessibility: layout reflows but the DOM order is unchanged, so reading and tab order stay stable across the breakpoint. Replaces: element-width JavaScript and ResizeObserver wiring for what is fundamentally a styling concern.

View source — the container-query CSSplatform.scss

The .cq-outer declares container-type so it becomes a query container; the .cq-card rules inside resolve @container against that element's width, not the viewport. The slider only changes the container's inline-size — the breakpoint flip is pure CSS.

css

:host {
  display: block;
}

.demo-note {
  margin: 0;
  color: var(--foo-color-text-muted);
  line-height: var(--foo-leading-normal);
  max-width: 64rem;

  &--small {
    font-size: var(--foo-text-sm);
    margin-top: var(--foo-space-4);
  }
}

kbd {
  font-family: var(--foo-font-mono);
  font-size: 0.85em;
  padding: 0.1em 0.4em;
  border: 1px solid var(--foo-color-border-strong);
  border-bottom-width: 2px;
  border-radius: var(--foo-radius-sm);
  background: var(--foo-color-surface-sunken);
  color: var(--foo-color-text);
}

/* ---- 1. Container queries -------------------------------------------------- */

.cq-range {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-2);
  margin: var(--foo-space-4) 0;
  color: var(--foo-color-text-muted);
  font-size: var(--foo-text-sm);

  input[type='range'] {
    accent-color: var(--foo-color-brand);
    max-inline-size: 28rem;
  }
}

/* The resizable container. `container-type: inline-size` makes it a query container; the
   card inside resolves @container against THIS element's width, not the viewport. */
.cq-outer {
  container-type: inline-size;
  min-inline-size: 12rem;
  border: 1px dashed var(--foo-color-border-strong);
  border-radius: var(--foo-radius-lg);
  padding: var(--foo-space-3);
  background: var(--foo-color-surface-sunken);
}

.cq-card {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-3);
  padding: var(--foo-space-4);
  border-radius: var(--foo-radius-md);
  background: var(--foo-color-surface-raised);
  box-shadow: var(--foo-shadow-md);
}

.cq-card__media {
  display: grid;
  place-items: center;
  font-size: var(--foo-text-2xl);
  min-block-size: 4rem;
  border-radius: var(--foo-radius-sm);
  background: var(--foo-color-surface-sunken);
  color: var(--foo-color-text-muted);
}

.cq-card__title {
  margin: 0;
  font-size: var(--foo-text-lg);
  font-weight: var(--foo-weight-semibold);
}

.cq-card__text {
  margin: 0;
  color: var(--foo-color-text-muted);
  line-height: var(--foo-leading-normal);
}

.cq-card__state {
  margin: 0;
  font-size: var(--foo-text-sm);
  color: var(--foo-color-text-muted);
}

/* The breakpoint is on the CONTAINER, not the viewport. Above 28rem the card goes inline. */
@container (min-width: 28rem) {
  .cq-card {
    display: grid;
    grid-template-columns: 8rem 1fr;
    align-items: center;
  }

  .cq-card__media {
    block-size: 100%;
  }
}

/* ---- 2. :has() ------------------------------------------------------------- */

.has-form {
  margin-top: var(--foo-space-4);
  max-inline-size: 28rem;
}

.has-field {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-2);
  padding: var(--foo-space-4);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-lg);
  background: var(--foo-color-surface);
  transition: background var(--foo-transition-fast), border-color var(--foo-transition-fast);

  label {
    font-weight: var(--foo-weight-medium);
  }

  input {
    padding: var(--foo-space-2) var(--foo-space-3);
    border: 1px solid var(--foo-color-border-strong);
    border-radius: var(--foo-radius-sm);
    background: var(--foo-color-surface-raised);
    color: var(--foo-color-text);
    font: inherit;
  }

  input:focus-visible {
    outline: var(--foo-focus-ring);
    outline-offset: 2px;
  }
}

/* The parent tints itself when it CONTAINS an invalid input — pure CSS, no class toggling. */
.has-field:has(input[aria-invalid='true']) {
  border-color: var(--foo-color-danger);
  background: var(--foo-color-danger-bg);
}

.has-field:has(input:not([aria-invalid='true'])) {
  border-color: var(--foo-color-success);
  background: var(--foo-color-success-bg);
}

.has-field__error {
  margin: 0;
  font-size: var(--foo-text-sm);
  /* Danger as text on its own tint — the AAA -text shade clears 7:1 here. */
  color: var(--foo-color-danger-text);
}

.has-field__ok {
  margin: 0;
  font-size: var(--foo-text-sm);
  color: var(--foo-color-success-text);
}

/* ---- 3. Cascade layers ----------------------------------------------------- */

.layer-pre {
  margin: var(--foo-space-4) 0;
  padding: var(--foo-space-3) var(--foo-space-4);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-md);
  background: var(--foo-color-surface-sunken);
  overflow-x: auto;

  code {
    font-size: var(--foo-text-sm);
  }
}

.layer-stack {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-2);
  max-inline-size: 32rem;
}

.layer-stack__item {
  display: flex;
  align-items: center;
  gap: var(--foo-space-3);
  padding: var(--foo-space-3);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-md);
  background: var(--foo-color-surface);
  color: var(--foo-color-text-muted);
  font-size: var(--foo-text-sm);
}

/* ---- 4. details / summary -------------------------------------------------- */

.disclosure-group {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-2);
  margin-top: var(--foo-space-4);
  max-inline-size: 44rem;
}

.disclosure {
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-md);
  background: var(--foo-color-surface);
  overflow: hidden;
}

.disclosure__summary {
  cursor: pointer;
  padding: var(--foo-space-3) var(--foo-space-4);
  font-weight: var(--foo-weight-medium);
  list-style-position: inside;

  &:focus-visible {
    outline: var(--foo-focus-ring);
    outline-offset: -2px;
  }
}

.disclosure__body {
  padding: 0 var(--foo-space-4) var(--foo-space-4);
  color: var(--foo-color-text-muted);
  line-height: var(--foo-leading-normal);
}

/* ---- 5. Popover API -------------------------------------------------------- */

.popover-trigger {
  margin-top: var(--foo-space-4);
  padding: var(--foo-space-2) var(--foo-space-4);
  border: 1px solid var(--foo-color-border-strong);
  border-radius: var(--foo-radius-sm);
  background: var(--foo-color-brand-fill);
  color: var(--foo-color-text-on-brand);
  font: inherit;
  font-weight: var(--foo-weight-medium);
  cursor: pointer;

  &:hover {
    background: var(--foo-color-brand-fill-hover);
  }

  &:focus-visible {
    outline: var(--foo-focus-ring);
    outline-offset: 2px;
  }
}

.popover-panel {
  max-inline-size: 22rem;
  margin: auto;
  padding: var(--foo-space-4);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-lg);
  background: var(--foo-color-surface-raised);
  color: var(--foo-color-text);
  box-shadow: var(--foo-shadow-lg);

  p {
    margin: 0 0 var(--foo-space-2);

    &:last-child {
      margin-bottom: 0;
    }
  }
}

/* ---- 6. Subgrid ------------------------------------------------------------ */

.subgrid-row {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(14rem, 1fr));
  /* Three rows the children will inherit via subgrid: title / body / footer. */
  grid-template-rows: auto 1fr auto;
  gap: var(--foo-space-4);
  margin-top: var(--foo-space-4);
}

.subgrid-card {
  display: grid;
  grid-row: span 3;
  /* Inherit the parent's row tracks so rows align ACROSS cards. */
  grid-template-rows: subgrid;
  gap: var(--foo-space-2);
  padding: var(--foo-space-4);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-lg);
  background: var(--foo-color-surface);
}

.subgrid-card__title {
  margin: 0;
  font-size: var(--foo-text-lg);
  font-weight: var(--foo-weight-semibold);
}

.subgrid-card__body {
  margin: 0;
  color: var(--foo-color-text-muted);
  line-height: var(--foo-leading-normal);
}

.subgrid-card__footer {
  margin: 0;
  padding-block-start: var(--foo-space-2);
  border-block-start: 1px solid var(--foo-color-border);
  font-size: var(--foo-text-sm);
  font-weight: var(--foo-weight-medium);
  color: var(--foo-color-text-muted);
}

/* ---- 7. Fluid type + logical properties ------------------------------------ */

.fluid-demo {
  margin: var(--foo-space-4) 0;
  /* Scales between 1.25rem and 2.5rem with the viewport, clamped at both ends so it
     stays readable when zoomed and never overflows on wide screens. */
  font-size: clamp(1.25rem, 1rem + 2.5vw, 2.5rem);
  font-weight: var(--foo-weight-bold);
  line-height: var(--foo-leading-tight);
}

.logical-demo {
  display: flex;
  align-items: baseline;
  gap: var(--foo-space-3);
  padding-block: var(--foo-space-3);
  padding-inline: var(--foo-space-4);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-lg);
  background: var(--foo-color-surface);
}

.logical-demo__chip {
  /* margin-inline-start flips with writing direction — left in LTR, right in RTL. */
  margin-inline-start: var(--foo-space-2);
  padding: var(--foo-space-1) var(--foo-space-3);
  border-radius: var(--foo-radius-pill);
  background: var(--foo-color-brand-fill);
  color: var(--foo-color-text-on-brand);
  font-size: var(--foo-text-sm);
  font-weight: var(--foo-weight-medium);
  white-space: nowrap;
}

.logical-demo__note {
  margin: 0;
  color: var(--foo-color-text-muted);
  line-height: var(--foo-leading-normal);
  font-size: var(--foo-text-sm);
}

/* ---- 8. picture / responsive images ---------------------------------------- */

.picture-demo {
  display: block;
  margin-top: var(--foo-space-4);
}

.picture-demo__img {
  display: block;
  /* The intrinsic width/height attributes give the box an aspect ratio up front; capping the
     rendered size keeps it tidy while the browser still reserves space to prevent CLS. */
  max-inline-size: 100%;
  block-size: auto;
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-lg);
  background: var(--foo-color-surface-sunken);
}

/* ---- 9. Native constraint validation --------------------------------------- */

.cv-form {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-4);
  margin-top: var(--foo-space-4);
  max-inline-size: 28rem;
}

.cv-field {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-2);

  label {
    font-weight: var(--foo-weight-medium);
  }

  input {
    padding: var(--foo-space-2) var(--foo-space-3);
    border: 1px solid var(--foo-color-border-strong);
    border-radius: var(--foo-radius-sm);
    background: var(--foo-color-surface-raised);
    color: var(--foo-color-text);
    font: inherit;
  }

  input:focus-visible {
    outline: var(--foo-focus-ring);
    outline-offset: 2px;
  }

  /* :user-invalid only tints after the field has been interacted with and submitted, so the
     form doesn't look broken before the user has touched it. */
  input:user-invalid {
    border-color: var(--foo-color-danger);
    background: var(--foo-color-danger-bg);
  }

  input:user-valid {
    border-color: var(--foo-color-success);
  }
}

.cv-field__hint {
  margin: 0;
  font-size: var(--foo-text-sm);
  color: var(--foo-color-text-muted);
}

.cv-submit {
  align-self: flex-start;
  padding: var(--foo-space-2) var(--foo-space-4);
  border: 1px solid var(--foo-color-border-strong);
  border-radius: var(--foo-radius-sm);
  background: var(--foo-color-brand-fill);
  color: var(--foo-color-text-on-brand);
  font: inherit;
  font-weight: var(--foo-weight-medium);
  cursor: pointer;

  &:hover {
    background: var(--foo-color-brand-fill-hover);
  }

  &:focus-visible {
    outline: var(--foo-focus-ring);
    outline-offset: 2px;
  }
}

.cv-status {
  margin-top: var(--foo-space-4);
  max-inline-size: 32rem;

  /* Empty by default; only render the box once there's something to announce. */
  &:empty {
    margin-top: 0;
  }
}

.cv-status__ok {
  margin: 0;
  font-size: var(--foo-text-sm);
  color: var(--foo-color-success-text);
}

.cv-status__heading {
  margin: 0 0 var(--foo-space-2);
  font-size: var(--foo-text-sm);
  font-weight: var(--foo-weight-medium);
  /* Danger as text — the AAA -text shade clears 7:1 on the surface here. */
  color: var(--foo-color-danger-text);
}

.cv-status__list {
  margin: 0;
  padding-inline-start: var(--foo-space-5);
  color: var(--foo-color-danger-text);
  font-size: var(--foo-text-sm);
  line-height: var(--foo-leading-normal);

  li + li {
    margin-block-start: var(--foo-space-1);
  }
}

@media (prefers-reduced-motion: reduce) {
  .has-field {
    transition: none;
  }
}

The parent selector — `:has()`

For years CSS could only style down the tree. :has() lets a parent react to a child's state — here a field group tints itself when it contains an invalid input, with no class toggling in component code. Clear the field, or remove the @, to see the group go into its error state.

Accessibility: :has() drives the visual tint only — the real error signal is still aria-invalid plus a linked aria-describedby message, so colour is never the sole channel. Replaces: JS that adds an .is-invalid class to a wrapper on every keystroke.

View source — the :has() field CSSplatform.scss

The .has-field rule keys off :has(input[aria-invalid='true']) — the parent tints itself from its child's validity with no class toggling in component code. The styling reads the same aria-invalid the screen reader does, so the visual and the accessible signal can't drift.

css

:host {
  display: block;
}

.demo-note {
  margin: 0;
  color: var(--foo-color-text-muted);
  line-height: var(--foo-leading-normal);
  max-width: 64rem;

  &--small {
    font-size: var(--foo-text-sm);
    margin-top: var(--foo-space-4);
  }
}

kbd {
  font-family: var(--foo-font-mono);
  font-size: 0.85em;
  padding: 0.1em 0.4em;
  border: 1px solid var(--foo-color-border-strong);
  border-bottom-width: 2px;
  border-radius: var(--foo-radius-sm);
  background: var(--foo-color-surface-sunken);
  color: var(--foo-color-text);
}

/* ---- 1. Container queries -------------------------------------------------- */

.cq-range {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-2);
  margin: var(--foo-space-4) 0;
  color: var(--foo-color-text-muted);
  font-size: var(--foo-text-sm);

  input[type='range'] {
    accent-color: var(--foo-color-brand);
    max-inline-size: 28rem;
  }
}

/* The resizable container. `container-type: inline-size` makes it a query container; the
   card inside resolves @container against THIS element's width, not the viewport. */
.cq-outer {
  container-type: inline-size;
  min-inline-size: 12rem;
  border: 1px dashed var(--foo-color-border-strong);
  border-radius: var(--foo-radius-lg);
  padding: var(--foo-space-3);
  background: var(--foo-color-surface-sunken);
}

.cq-card {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-3);
  padding: var(--foo-space-4);
  border-radius: var(--foo-radius-md);
  background: var(--foo-color-surface-raised);
  box-shadow: var(--foo-shadow-md);
}

.cq-card__media {
  display: grid;
  place-items: center;
  font-size: var(--foo-text-2xl);
  min-block-size: 4rem;
  border-radius: var(--foo-radius-sm);
  background: var(--foo-color-surface-sunken);
  color: var(--foo-color-text-muted);
}

.cq-card__title {
  margin: 0;
  font-size: var(--foo-text-lg);
  font-weight: var(--foo-weight-semibold);
}

.cq-card__text {
  margin: 0;
  color: var(--foo-color-text-muted);
  line-height: var(--foo-leading-normal);
}

.cq-card__state {
  margin: 0;
  font-size: var(--foo-text-sm);
  color: var(--foo-color-text-muted);
}

/* The breakpoint is on the CONTAINER, not the viewport. Above 28rem the card goes inline. */
@container (min-width: 28rem) {
  .cq-card {
    display: grid;
    grid-template-columns: 8rem 1fr;
    align-items: center;
  }

  .cq-card__media {
    block-size: 100%;
  }
}

/* ---- 2. :has() ------------------------------------------------------------- */

.has-form {
  margin-top: var(--foo-space-4);
  max-inline-size: 28rem;
}

.has-field {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-2);
  padding: var(--foo-space-4);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-lg);
  background: var(--foo-color-surface);
  transition: background var(--foo-transition-fast), border-color var(--foo-transition-fast);

  label {
    font-weight: var(--foo-weight-medium);
  }

  input {
    padding: var(--foo-space-2) var(--foo-space-3);
    border: 1px solid var(--foo-color-border-strong);
    border-radius: var(--foo-radius-sm);
    background: var(--foo-color-surface-raised);
    color: var(--foo-color-text);
    font: inherit;
  }

  input:focus-visible {
    outline: var(--foo-focus-ring);
    outline-offset: 2px;
  }
}

/* The parent tints itself when it CONTAINS an invalid input — pure CSS, no class toggling. */
.has-field:has(input[aria-invalid='true']) {
  border-color: var(--foo-color-danger);
  background: var(--foo-color-danger-bg);
}

.has-field:has(input:not([aria-invalid='true'])) {
  border-color: var(--foo-color-success);
  background: var(--foo-color-success-bg);
}

.has-field__error {
  margin: 0;
  font-size: var(--foo-text-sm);
  /* Danger as text on its own tint — the AAA -text shade clears 7:1 here. */
  color: var(--foo-color-danger-text);
}

.has-field__ok {
  margin: 0;
  font-size: var(--foo-text-sm);
  color: var(--foo-color-success-text);
}

/* ---- 3. Cascade layers ----------------------------------------------------- */

.layer-pre {
  margin: var(--foo-space-4) 0;
  padding: var(--foo-space-3) var(--foo-space-4);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-md);
  background: var(--foo-color-surface-sunken);
  overflow-x: auto;

  code {
    font-size: var(--foo-text-sm);
  }
}

.layer-stack {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-2);
  max-inline-size: 32rem;
}

.layer-stack__item {
  display: flex;
  align-items: center;
  gap: var(--foo-space-3);
  padding: var(--foo-space-3);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-md);
  background: var(--foo-color-surface);
  color: var(--foo-color-text-muted);
  font-size: var(--foo-text-sm);
}

/* ---- 4. details / summary -------------------------------------------------- */

.disclosure-group {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-2);
  margin-top: var(--foo-space-4);
  max-inline-size: 44rem;
}

.disclosure {
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-md);
  background: var(--foo-color-surface);
  overflow: hidden;
}

.disclosure__summary {
  cursor: pointer;
  padding: var(--foo-space-3) var(--foo-space-4);
  font-weight: var(--foo-weight-medium);
  list-style-position: inside;

  &:focus-visible {
    outline: var(--foo-focus-ring);
    outline-offset: -2px;
  }
}

.disclosure__body {
  padding: 0 var(--foo-space-4) var(--foo-space-4);
  color: var(--foo-color-text-muted);
  line-height: var(--foo-leading-normal);
}

/* ---- 5. Popover API -------------------------------------------------------- */

.popover-trigger {
  margin-top: var(--foo-space-4);
  padding: var(--foo-space-2) var(--foo-space-4);
  border: 1px solid var(--foo-color-border-strong);
  border-radius: var(--foo-radius-sm);
  background: var(--foo-color-brand-fill);
  color: var(--foo-color-text-on-brand);
  font: inherit;
  font-weight: var(--foo-weight-medium);
  cursor: pointer;

  &:hover {
    background: var(--foo-color-brand-fill-hover);
  }

  &:focus-visible {
    outline: var(--foo-focus-ring);
    outline-offset: 2px;
  }
}

.popover-panel {
  max-inline-size: 22rem;
  margin: auto;
  padding: var(--foo-space-4);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-lg);
  background: var(--foo-color-surface-raised);
  color: var(--foo-color-text);
  box-shadow: var(--foo-shadow-lg);

  p {
    margin: 0 0 var(--foo-space-2);

    &:last-child {
      margin-bottom: 0;
    }
  }
}

/* ---- 6. Subgrid ------------------------------------------------------------ */

.subgrid-row {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(14rem, 1fr));
  /* Three rows the children will inherit via subgrid: title / body / footer. */
  grid-template-rows: auto 1fr auto;
  gap: var(--foo-space-4);
  margin-top: var(--foo-space-4);
}

.subgrid-card {
  display: grid;
  grid-row: span 3;
  /* Inherit the parent's row tracks so rows align ACROSS cards. */
  grid-template-rows: subgrid;
  gap: var(--foo-space-2);
  padding: var(--foo-space-4);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-lg);
  background: var(--foo-color-surface);
}

.subgrid-card__title {
  margin: 0;
  font-size: var(--foo-text-lg);
  font-weight: var(--foo-weight-semibold);
}

.subgrid-card__body {
  margin: 0;
  color: var(--foo-color-text-muted);
  line-height: var(--foo-leading-normal);
}

.subgrid-card__footer {
  margin: 0;
  padding-block-start: var(--foo-space-2);
  border-block-start: 1px solid var(--foo-color-border);
  font-size: var(--foo-text-sm);
  font-weight: var(--foo-weight-medium);
  color: var(--foo-color-text-muted);
}

/* ---- 7. Fluid type + logical properties ------------------------------------ */

.fluid-demo {
  margin: var(--foo-space-4) 0;
  /* Scales between 1.25rem and 2.5rem with the viewport, clamped at both ends so it
     stays readable when zoomed and never overflows on wide screens. */
  font-size: clamp(1.25rem, 1rem + 2.5vw, 2.5rem);
  font-weight: var(--foo-weight-bold);
  line-height: var(--foo-leading-tight);
}

.logical-demo {
  display: flex;
  align-items: baseline;
  gap: var(--foo-space-3);
  padding-block: var(--foo-space-3);
  padding-inline: var(--foo-space-4);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-lg);
  background: var(--foo-color-surface);
}

.logical-demo__chip {
  /* margin-inline-start flips with writing direction — left in LTR, right in RTL. */
  margin-inline-start: var(--foo-space-2);
  padding: var(--foo-space-1) var(--foo-space-3);
  border-radius: var(--foo-radius-pill);
  background: var(--foo-color-brand-fill);
  color: var(--foo-color-text-on-brand);
  font-size: var(--foo-text-sm);
  font-weight: var(--foo-weight-medium);
  white-space: nowrap;
}

.logical-demo__note {
  margin: 0;
  color: var(--foo-color-text-muted);
  line-height: var(--foo-leading-normal);
  font-size: var(--foo-text-sm);
}

/* ---- 8. picture / responsive images ---------------------------------------- */

.picture-demo {
  display: block;
  margin-top: var(--foo-space-4);
}

.picture-demo__img {
  display: block;
  /* The intrinsic width/height attributes give the box an aspect ratio up front; capping the
     rendered size keeps it tidy while the browser still reserves space to prevent CLS. */
  max-inline-size: 100%;
  block-size: auto;
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-lg);
  background: var(--foo-color-surface-sunken);
}

/* ---- 9. Native constraint validation --------------------------------------- */

.cv-form {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-4);
  margin-top: var(--foo-space-4);
  max-inline-size: 28rem;
}

.cv-field {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-2);

  label {
    font-weight: var(--foo-weight-medium);
  }

  input {
    padding: var(--foo-space-2) var(--foo-space-3);
    border: 1px solid var(--foo-color-border-strong);
    border-radius: var(--foo-radius-sm);
    background: var(--foo-color-surface-raised);
    color: var(--foo-color-text);
    font: inherit;
  }

  input:focus-visible {
    outline: var(--foo-focus-ring);
    outline-offset: 2px;
  }

  /* :user-invalid only tints after the field has been interacted with and submitted, so the
     form doesn't look broken before the user has touched it. */
  input:user-invalid {
    border-color: var(--foo-color-danger);
    background: var(--foo-color-danger-bg);
  }

  input:user-valid {
    border-color: var(--foo-color-success);
  }
}

.cv-field__hint {
  margin: 0;
  font-size: var(--foo-text-sm);
  color: var(--foo-color-text-muted);
}

.cv-submit {
  align-self: flex-start;
  padding: var(--foo-space-2) var(--foo-space-4);
  border: 1px solid var(--foo-color-border-strong);
  border-radius: var(--foo-radius-sm);
  background: var(--foo-color-brand-fill);
  color: var(--foo-color-text-on-brand);
  font: inherit;
  font-weight: var(--foo-weight-medium);
  cursor: pointer;

  &:hover {
    background: var(--foo-color-brand-fill-hover);
  }

  &:focus-visible {
    outline: var(--foo-focus-ring);
    outline-offset: 2px;
  }
}

.cv-status {
  margin-top: var(--foo-space-4);
  max-inline-size: 32rem;

  /* Empty by default; only render the box once there's something to announce. */
  &:empty {
    margin-top: 0;
  }
}

.cv-status__ok {
  margin: 0;
  font-size: var(--foo-text-sm);
  color: var(--foo-color-success-text);
}

.cv-status__heading {
  margin: 0 0 var(--foo-space-2);
  font-size: var(--foo-text-sm);
  font-weight: var(--foo-weight-medium);
  /* Danger as text — the AAA -text shade clears 7:1 on the surface here. */
  color: var(--foo-color-danger-text);
}

.cv-status__list {
  margin: 0;
  padding-inline-start: var(--foo-space-5);
  color: var(--foo-color-danger-text);
  font-size: var(--foo-text-sm);
  line-height: var(--foo-leading-normal);

  li + li {
    margin-block-start: var(--foo-space-1);
  }
}

@media (prefers-reduced-motion: reduce) {
  .has-field {
    transition: none;
  }
}

Cascade layers — `@layer`

Specificity wars end when the cascade is explicit. @layer lets you name the order once; a later layer always wins over an earlier one regardless of selector specificity, so a single-class utility can override a complex component rule without !important. The order below is declared first, then layers fill in any sequence.

@layer reset, tokens, components, utilities;

reset base normalisation

tokens design variables

components @foo/ui rules

utilities wins last

No accessibility surface of its own — it's an authoring-time tool. Replaces: !important stacks, deeply chained selectors, and source-order fragility.

Disclosure — `<details>` / `<summary>`

A disclosure widget with zero JavaScript. <details> is keyboard operable, exposes expanded/collapsed state to assistive tech, and is even findable by in-page search in modern browsers. Use Tab to reach a summary, then Enter or Space to toggle it.

What does this replace?

A hand-rolled accordion: a button, an aria-expanded attribute you keep in sync, a click handler, and height animation. The platform gives you all of that for free, correctly.

Is it accessible by default?

Yes — focusable, toggles on Enter/Space, and announces its open state. Keep the summary text meaningful and avoid nesting interactive controls directly inside the summary line.

A native popover: a popover attribute on the panel and a popovertarget on the button. The platform handles top-layer stacking, light-dismiss (click outside or Escape), and focus — no z-index management or outside-click listeners.

Popover API not detected

This browser (or the prerendered server pass) doesn't report support for the popover attribute. In production you'd progressively enhance: render an always-visible disclosure as the baseline and upgrade to a popover only when supported.

Accessibility: give the panel a role and a label, and ensure the trigger's meaning is clear. Replaces: bespoke overlay libraries, focus-trap utilities, and z-index ladders.

View source — the popover panel CSSplatform.scss

The .popover-panel styling is all the CSS this needs — the platform handles the top-layer placement, light-dismiss, and focus once the popover and popovertarget attributes wire the panel to its trigger. No z-index, no outside-click listener, no overlay container.

css

:host {
  display: block;
}

.demo-note {
  margin: 0;
  color: var(--foo-color-text-muted);
  line-height: var(--foo-leading-normal);
  max-width: 64rem;

  &--small {
    font-size: var(--foo-text-sm);
    margin-top: var(--foo-space-4);
  }
}

kbd {
  font-family: var(--foo-font-mono);
  font-size: 0.85em;
  padding: 0.1em 0.4em;
  border: 1px solid var(--foo-color-border-strong);
  border-bottom-width: 2px;
  border-radius: var(--foo-radius-sm);
  background: var(--foo-color-surface-sunken);
  color: var(--foo-color-text);
}

/* ---- 1. Container queries -------------------------------------------------- */

.cq-range {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-2);
  margin: var(--foo-space-4) 0;
  color: var(--foo-color-text-muted);
  font-size: var(--foo-text-sm);

  input[type='range'] {
    accent-color: var(--foo-color-brand);
    max-inline-size: 28rem;
  }
}

/* The resizable container. `container-type: inline-size` makes it a query container; the
   card inside resolves @container against THIS element's width, not the viewport. */
.cq-outer {
  container-type: inline-size;
  min-inline-size: 12rem;
  border: 1px dashed var(--foo-color-border-strong);
  border-radius: var(--foo-radius-lg);
  padding: var(--foo-space-3);
  background: var(--foo-color-surface-sunken);
}

.cq-card {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-3);
  padding: var(--foo-space-4);
  border-radius: var(--foo-radius-md);
  background: var(--foo-color-surface-raised);
  box-shadow: var(--foo-shadow-md);
}

.cq-card__media {
  display: grid;
  place-items: center;
  font-size: var(--foo-text-2xl);
  min-block-size: 4rem;
  border-radius: var(--foo-radius-sm);
  background: var(--foo-color-surface-sunken);
  color: var(--foo-color-text-muted);
}

.cq-card__title {
  margin: 0;
  font-size: var(--foo-text-lg);
  font-weight: var(--foo-weight-semibold);
}

.cq-card__text {
  margin: 0;
  color: var(--foo-color-text-muted);
  line-height: var(--foo-leading-normal);
}

.cq-card__state {
  margin: 0;
  font-size: var(--foo-text-sm);
  color: var(--foo-color-text-muted);
}

/* The breakpoint is on the CONTAINER, not the viewport. Above 28rem the card goes inline. */
@container (min-width: 28rem) {
  .cq-card {
    display: grid;
    grid-template-columns: 8rem 1fr;
    align-items: center;
  }

  .cq-card__media {
    block-size: 100%;
  }
}

/* ---- 2. :has() ------------------------------------------------------------- */

.has-form {
  margin-top: var(--foo-space-4);
  max-inline-size: 28rem;
}

.has-field {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-2);
  padding: var(--foo-space-4);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-lg);
  background: var(--foo-color-surface);
  transition: background var(--foo-transition-fast), border-color var(--foo-transition-fast);

  label {
    font-weight: var(--foo-weight-medium);
  }

  input {
    padding: var(--foo-space-2) var(--foo-space-3);
    border: 1px solid var(--foo-color-border-strong);
    border-radius: var(--foo-radius-sm);
    background: var(--foo-color-surface-raised);
    color: var(--foo-color-text);
    font: inherit;
  }

  input:focus-visible {
    outline: var(--foo-focus-ring);
    outline-offset: 2px;
  }
}

/* The parent tints itself when it CONTAINS an invalid input — pure CSS, no class toggling. */
.has-field:has(input[aria-invalid='true']) {
  border-color: var(--foo-color-danger);
  background: var(--foo-color-danger-bg);
}

.has-field:has(input:not([aria-invalid='true'])) {
  border-color: var(--foo-color-success);
  background: var(--foo-color-success-bg);
}

.has-field__error {
  margin: 0;
  font-size: var(--foo-text-sm);
  /* Danger as text on its own tint — the AAA -text shade clears 7:1 here. */
  color: var(--foo-color-danger-text);
}

.has-field__ok {
  margin: 0;
  font-size: var(--foo-text-sm);
  color: var(--foo-color-success-text);
}

/* ---- 3. Cascade layers ----------------------------------------------------- */

.layer-pre {
  margin: var(--foo-space-4) 0;
  padding: var(--foo-space-3) var(--foo-space-4);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-md);
  background: var(--foo-color-surface-sunken);
  overflow-x: auto;

  code {
    font-size: var(--foo-text-sm);
  }
}

.layer-stack {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-2);
  max-inline-size: 32rem;
}

.layer-stack__item {
  display: flex;
  align-items: center;
  gap: var(--foo-space-3);
  padding: var(--foo-space-3);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-md);
  background: var(--foo-color-surface);
  color: var(--foo-color-text-muted);
  font-size: var(--foo-text-sm);
}

/* ---- 4. details / summary -------------------------------------------------- */

.disclosure-group {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-2);
  margin-top: var(--foo-space-4);
  max-inline-size: 44rem;
}

.disclosure {
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-md);
  background: var(--foo-color-surface);
  overflow: hidden;
}

.disclosure__summary {
  cursor: pointer;
  padding: var(--foo-space-3) var(--foo-space-4);
  font-weight: var(--foo-weight-medium);
  list-style-position: inside;

  &:focus-visible {
    outline: var(--foo-focus-ring);
    outline-offset: -2px;
  }
}

.disclosure__body {
  padding: 0 var(--foo-space-4) var(--foo-space-4);
  color: var(--foo-color-text-muted);
  line-height: var(--foo-leading-normal);
}

/* ---- 5. Popover API -------------------------------------------------------- */

.popover-trigger {
  margin-top: var(--foo-space-4);
  padding: var(--foo-space-2) var(--foo-space-4);
  border: 1px solid var(--foo-color-border-strong);
  border-radius: var(--foo-radius-sm);
  background: var(--foo-color-brand-fill);
  color: var(--foo-color-text-on-brand);
  font: inherit;
  font-weight: var(--foo-weight-medium);
  cursor: pointer;

  &:hover {
    background: var(--foo-color-brand-fill-hover);
  }

  &:focus-visible {
    outline: var(--foo-focus-ring);
    outline-offset: 2px;
  }
}

.popover-panel {
  max-inline-size: 22rem;
  margin: auto;
  padding: var(--foo-space-4);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-lg);
  background: var(--foo-color-surface-raised);
  color: var(--foo-color-text);
  box-shadow: var(--foo-shadow-lg);

  p {
    margin: 0 0 var(--foo-space-2);

    &:last-child {
      margin-bottom: 0;
    }
  }
}

/* ---- 6. Subgrid ------------------------------------------------------------ */

.subgrid-row {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(14rem, 1fr));
  /* Three rows the children will inherit via subgrid: title / body / footer. */
  grid-template-rows: auto 1fr auto;
  gap: var(--foo-space-4);
  margin-top: var(--foo-space-4);
}

.subgrid-card {
  display: grid;
  grid-row: span 3;
  /* Inherit the parent's row tracks so rows align ACROSS cards. */
  grid-template-rows: subgrid;
  gap: var(--foo-space-2);
  padding: var(--foo-space-4);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-lg);
  background: var(--foo-color-surface);
}

.subgrid-card__title {
  margin: 0;
  font-size: var(--foo-text-lg);
  font-weight: var(--foo-weight-semibold);
}

.subgrid-card__body {
  margin: 0;
  color: var(--foo-color-text-muted);
  line-height: var(--foo-leading-normal);
}

.subgrid-card__footer {
  margin: 0;
  padding-block-start: var(--foo-space-2);
  border-block-start: 1px solid var(--foo-color-border);
  font-size: var(--foo-text-sm);
  font-weight: var(--foo-weight-medium);
  color: var(--foo-color-text-muted);
}

/* ---- 7. Fluid type + logical properties ------------------------------------ */

.fluid-demo {
  margin: var(--foo-space-4) 0;
  /* Scales between 1.25rem and 2.5rem with the viewport, clamped at both ends so it
     stays readable when zoomed and never overflows on wide screens. */
  font-size: clamp(1.25rem, 1rem + 2.5vw, 2.5rem);
  font-weight: var(--foo-weight-bold);
  line-height: var(--foo-leading-tight);
}

.logical-demo {
  display: flex;
  align-items: baseline;
  gap: var(--foo-space-3);
  padding-block: var(--foo-space-3);
  padding-inline: var(--foo-space-4);
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-lg);
  background: var(--foo-color-surface);
}

.logical-demo__chip {
  /* margin-inline-start flips with writing direction — left in LTR, right in RTL. */
  margin-inline-start: var(--foo-space-2);
  padding: var(--foo-space-1) var(--foo-space-3);
  border-radius: var(--foo-radius-pill);
  background: var(--foo-color-brand-fill);
  color: var(--foo-color-text-on-brand);
  font-size: var(--foo-text-sm);
  font-weight: var(--foo-weight-medium);
  white-space: nowrap;
}

.logical-demo__note {
  margin: 0;
  color: var(--foo-color-text-muted);
  line-height: var(--foo-leading-normal);
  font-size: var(--foo-text-sm);
}

/* ---- 8. picture / responsive images ---------------------------------------- */

.picture-demo {
  display: block;
  margin-top: var(--foo-space-4);
}

.picture-demo__img {
  display: block;
  /* The intrinsic width/height attributes give the box an aspect ratio up front; capping the
     rendered size keeps it tidy while the browser still reserves space to prevent CLS. */
  max-inline-size: 100%;
  block-size: auto;
  border: 1px solid var(--foo-color-border);
  border-radius: var(--foo-radius-lg);
  background: var(--foo-color-surface-sunken);
}

/* ---- 9. Native constraint validation --------------------------------------- */

.cv-form {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-4);
  margin-top: var(--foo-space-4);
  max-inline-size: 28rem;
}

.cv-field {
  display: flex;
  flex-direction: column;
  gap: var(--foo-space-2);

  label {
    font-weight: var(--foo-weight-medium);
  }

  input {
    padding: var(--foo-space-2) var(--foo-space-3);
    border: 1px solid var(--foo-color-border-strong);
    border-radius: var(--foo-radius-sm);
    background: var(--foo-color-surface-raised);
    color: var(--foo-color-text);
    font: inherit;
  }

  input:focus-visible {
    outline: var(--foo-focus-ring);
    outline-offset: 2px;
  }

  /* :user-invalid only tints after the field has been interacted with and submitted, so the
     form doesn't look broken before the user has touched it. */
  input:user-invalid {
    border-color: var(--foo-color-danger);
    background: var(--foo-color-danger-bg);
  }

  input:user-valid {
    border-color: var(--foo-color-success);
  }
}

.cv-field__hint {
  margin: 0;
  font-size: var(--foo-text-sm);
  color: var(--foo-color-text-muted);
}

.cv-submit {
  align-self: flex-start;
  padding: var(--foo-space-2) var(--foo-space-4);
  border: 1px solid var(--foo-color-border-strong);
  border-radius: var(--foo-radius-sm);
  background: var(--foo-color-brand-fill);
  color: var(--foo-color-text-on-brand);
  font: inherit;
  font-weight: var(--foo-weight-medium);
  cursor: pointer;

  &:hover {
    background: var(--foo-color-brand-fill-hover);
  }

  &:focus-visible {
    outline: var(--foo-focus-ring);
    outline-offset: 2px;
  }
}

.cv-status {
  margin-top: var(--foo-space-4);
  max-inline-size: 32rem;

  /* Empty by default; only render the box once there's something to announce. */
  &:empty {
    margin-top: 0;
  }
}

.cv-status__ok {
  margin: 0;
  font-size: var(--foo-text-sm);
  color: var(--foo-color-success-text);
}

.cv-status__heading {
  margin: 0 0 var(--foo-space-2);
  font-size: var(--foo-text-sm);
  font-weight: var(--foo-weight-medium);
  /* Danger as text — the AAA -text shade clears 7:1 on the surface here. */
  color: var(--foo-color-danger-text);
}

.cv-status__list {
  margin: 0;
  padding-inline-start: var(--foo-space-5);
  color: var(--foo-color-danger-text);
  font-size: var(--foo-text-sm);
  line-height: var(--foo-leading-normal);

  li + li {
    margin-block-start: var(--foo-space-1);
  }
}

@media (prefers-reduced-motion: reduce) {
  .has-field {
    transition: none;
  }
}

Subgrid

Cards in a row rarely have matching content lengths, so their internal rows drift out of alignment. grid-template-rows: subgrid lets each card inherit the parent grid's row tracks, so titles, bodies, and footers line up across cards — even when the body text differs. Note how the footers sit on one baseline despite uneven bodies.

Plan 1

A short description.

Plan 2

A noticeably longer description that wraps onto several lines so the body rows would otherwise be different heights across the row.

Plan 3

Medium-length copy that sits between the other two.

Accessibility & i18n: alignment is purely visual, so translated strings of any length keep their rows aligned without magic pixel heights. Replaces: equal-height JS and fixed min-height hacks that break under longer locales.

Fluid type with `clamp()` & logical properties

clamp(min, preferred, max) scales a value smoothly between two bounds, so type grows with the viewport without a stack of breakpoints — and stays within accessible limits at both ends. Logical properties (margin-inline, padding-block, inline-size) describe space relative to the writing direction, so the same CSS mirrors correctly in right-to-left locales.

This heading scales fluidly with the viewport.

يبدأ هنا

The chip above uses margin-inline-start, so in this RTL block it sits on the right — the same property that puts it on the left in LTR. No direction-specific overrides.

Accessibility: clamp the maximum so text still reflows when a user zooms or sets a large base font; never disable zoom. Replaces: breakpoint ladders for type, and the left/right property pairs that had to be flipped by hand for RTL.

Responsive images — `<picture>`

A plain <img> ships one source to everyone. <picture> lets the browser pick: art-direction by media (a tall crop on narrow screens, a wide crop on roomy ones — not just a scaled-down rectangle), and format negotiation by type so modern codecs win where supported and a universal <img> is the guaranteed fallback. The browser evaluates the sources top-to-bottom and stops at the first it can use; the <img> still carries the alt, width, and height.

Resize the window across 40rem to see the crop change. The intrinsic width/height let the browser reserve the box before the bytes arrive, so there's no layout shift (CLS). Accessibility: the alt lives on the <img> and applies whichever source loads. Replaces: JS that swaps src on resize, and background-image hacks that hide the picture from assistive tech and search.

Native form validation — the Constraint Validation API

The browser already knows how to validate. required, type="email", pattern, and minlength declare the rules in markup; :invalid / :valid style the fields; and on submit the page reads each field's own validationMessage and validity object to surface errors — no Angular forms, no hand-written regex in component code. Submit empty, or with a bad email, to see the native messages.

Accessibility: the validity colour is backed by a live region that reads the browser's own validationMessage, so the result is announced, not just shown — and the messages are localised by the browser for free. Validation runs on submit, not per keystroke, to avoid flagging a field the user hasn't finished. Replaces: a forms library plus bespoke error copy for rules the platform already enforces.

Every feature here is shipping in evergreen browsers today. The throughline: push layout, state reflection, and overlays down into the platform, and reserve JavaScript for genuine behaviour — less code to ship, fewer accessibility regressions to own.

Container queries

Responsive to its container

The parent selector — :has()

Cascade layers — @layer

Disclosure — <details> / <summary>

Popover API