carousel/carousel.css
/* 
TODO: 
- option for cyclical cycling

NICE TO HAVE:
- variable sized items demo
- arrow placement options
- dynamic add/remove slide handling
- attribute change update handling
- web component
  - slots
  - async initialization
- js api
*/

:where(.gui-carousel) {
  --_carousel-item-size: 80%;
  --_carousel-gutters: max(4rem, calc((100% - var(--_carousel-item-size)) / 2));
  --_carousel-scrollbar-gutter: var(--size-6);
  --_carousel-pagination-size: var(--size-8);

  display: grid;
  grid-template-columns: [carousel-gutter] var(--_carousel-gutters) 1fr [carousel-gutter] var(--_carousel-gutters);
  grid-template-rows:
    [carousel-scroller] 1fr
    [carousel-pagination] var(--_carousel-pagination-size);

  &:focus-visible {
    outline-offset: -5px;
  }

  /* configuration handlers */
  &[carousel-pagination="gallery"] {
    --_carousel-pagination-size: var(--size-10);

    & > .gui-carousel--pagination {
      -webkit-mask-image: linear-gradient(to right, #0000 0%, #000 5%, 95%, #000, #0000);
    }
  }

  &[carousel-pagination="none"] {
    grid-template-rows: [carousel-scroller] 1fr;

    & > .gui-carousel--pagination {
      display: none;
    }
  }

  &[carousel-controls="none"] {
    grid-template-columns: 0 1fr 0;

    & > .gui-carousel--controls {
      display: none;
    }
  }

  &[carousel-scrollbar="none"] {
    --_carousel-pagination-size: var(--size-5);

    & > .gui-carousel--scroller {
      scrollbar-width: none;

      &::-webkit-scrollbar {
        display: none;
      }
    }

    & > .gui-carousel--pagination {
      place-self: start center;
    }
  }

  &[carousel-snapstop="always"] .gui-carousel--snap {
    scroll-snap-stop: always;
  }
}

:where(.gui-carousel--scroller) {
  grid-row: 1;
  grid-column: 1/-1;

  display: grid;
  grid-auto-columns: 100%;
  grid-auto-flow: column;
  align-items: center;
  gap: var(--_carousel-gutters);

  padding-block: var(--size-2) var(--_carousel-scrollbar-gutter);
  overflow-x: auto;
  overscroll-behavior-x: contain;
  scroll-snap-type: x mandatory;
  scroll-padding-inline: var(--_carousel-gutters);
  padding-inline: var(--_carousel-gutters);

  @media (--motionOK) {
    scroll-behavior: smooth;
  }
}

:where(.gui-carousel--snap) {
  scroll-snap-align: center;
}

:where(.gui-carousel--controls) {
  display: flex;
  justify-content: space-between;
  padding-inline: var(--_carousel-gutters);
  display: contents;

  & > .gui-carousel--control {
    margin-block-end: var(--_carousel-scrollbar-gutter);

    &:not([disabled="true"]):active {
      transform: scale(.95);
    }
  }
}

:where(.gui-carousel--control) {
  --_shadow-size: 0;
  --_shadow-highlight-light: hsl(0 0% 50% / 10%);
  --_shadow-highlight-dark: hsl(0 0% 100% / 20%);
  --_shadow-highlight: var(--_shadow-highlight-light);

  grid-row: 1;
  place-self: center;
  background: var(--surface-1);
  color: var(--text-2);
  inline-size: var(--size-8);
  aspect-ratio: var(--ratio-square);
  border-radius: var(--radius-round);
  box-shadow: 0 0 0 var(--_shadow-size) var(--_shadow-highlight);
  border: var(--border-size-1) solid transparent;
  text-indent: 10ch;
  padding: 0;
  overflow: hidden;
  z-index: var(--layer-1);
  transition: opacity .5s var(--ease-2) .5s;

  @media (--motionOK) {
    transition: 
      opacity .5s var(--ease-2) .5s, 
      transform .2s var(--ease-4),
      box-shadow .2s var(--ease-4),
      outline-offset 145ms var(--ease-2)
    ;
  }

  @media (--OSdark) {
    --_shadow-highlight: var(--_shadow-highlight-dark);
  }

  &:hover {
    --_shadow-size: 6px;
  }

  &.--previous {
    grid-column: 1;
  }

  &.--next {
    grid-column: 3;
  }

  @nest [dir="rtl"] & > svg {
    transform: rotateY(180deg);
  }

  &[disabled] {
    cursor: not-allowed;
    transition-delay: 0s;

    & > svg {
      opacity: .25;
    }
  }

  &:not([disabled]):is(:hover, :focus-visible) {
    color: var(--link);
  }

  &:not([disabled]) svg > path {
    @media (--motionOK) {
      --_transform: translateX(var(--_x)) scale(.95);
      transition: transform .5s var(--ease-squish-3);
      transform-origin: center center;
    }
  }

  &[aria-label="Next Item"]:not([disabled]):is(:hover, :focus-visible) svg > path {
    --_x: 2px;
    transform: var(--_transform);
  }

  &[aria-label="Previous Item"]:not([disabled]):is(:hover, :focus-visible) svg > path {
    --_x: -2px;
    transform: var(--_transform);
  }
}

:where(.gui-carousel--pagination) {
  grid-column: 1/-1;
  place-self: center;

  display: grid;
  grid-auto-flow: column;
  gap: var(--size-2);

  max-inline-size: 100%;
  overflow-x: auto;
  overscroll-behavior-x: contain;

  padding-block: var(--size-2);
  padding-inline: var(--size-4);

  scrollbar-width: none;

  &::-webkit-scrollbar {
    display: none;
  }

  @media (--motionOK) {
    scroll-behavior: smooth;
  }

  @nest [carousel-pagination="gallery"] & {
    margin-block-end: 0;
  }

  & > [aria-selected="true"] {
    background: var(--link);
  }

  & > [aria-selected="false"] {
    transform: scale(.75);
  }

  & > button {
    inline-size: var(--size-3);
    background-color: var(--surface-4);
    border: var(--border-size-1) solid transparent;

    &.--gallery {
      inline-size: var(--size-fluid-5);
      border-radius: var(--radius-2);
      border: none;
      background-origin: border-box;
      background-size: cover;
    }
  }
}

@keyframes gui-carousel--control-keypress {
  0%  { outline-offset: 5px }
  50% { outline-offset: 0; }
}