自訂文字裝飾

:where(.decorated) {
  /* Thickness is relative to the font size (0.08em is an example value), so the line is not too heavy
     on small text or too light on large text, and max() keeps it at least 1px so it never disappears. */
  text-decoration: underline solid currentColor max(1px, 0.08em);
  /* Use em so the gap scales with the font size. 0.2em clears most descenders; example value. */
  text-underline-offset: 0.2em;
}

/* Example hover state: a softer line that firms up on hover and focus. The 50% mix and
   0.12em thickness are example values. */
:where(.decorated) {
  text-decoration-color: color-mix(in srgb, currentColor, transparent);
}

:where(.decorated:is(:hover, :focus-visible)) {
  text-decoration-color: currentColor;
  text-decoration-thickness: max(1px, 0.12em);
}

/* Other lines are a different text-decoration-line on the same properties */
.decorated-over    { text-decoration-line: overline; }
.decorated-through { text-decoration-line: line-through; }

.decorated {
  /* auto: skip ink for most scripts (default). none: draw straight through.
     all: also skip ink in scripts where auto does not, such as Japanese or Chinese. */
  text-decoration-skip-ink: all;
}

.decorated {
  /* Spaces collapse by default, so there is nothing to skip. Preserve them when the
     leading or trailing spaces are meaningful, such as padded input values. */
  white-space: pre;
  /* none: decorate every space. all: skip every space, so each word is underlined separately.
     start | end | start end: skip only leading, trailing, or both. */
  text-decoration-skip-spaces: all;
}

/* Small insets soften the ends of the line; larger ones read as a deliberately short line.
   Both values are examples. */
.trimmed  { text-decoration-inset: var(--deco-inset, 0.1em); }
.extended { text-decoration-inset: calc(var(--deco-inset, 0.1em) * -1); }

.reveal {
  text-decoration: underline; /* Always visible, including for reduced motion */
}

/* Only hide and animate the line when the user has not asked for reduced motion. */
@media (prefers-reduced-motion: no-preference) {
  .reveal {
    /* Trim the end by the full inline size so the line starts with zero length. */
    text-decoration-inset: 0 100%;
    /* 150ms to 300ms feels responsive without lagging; ease-out starts quickly. Example values. */
    transition: text-decoration-inset var(--reveal-duration, 0.25s) ease-out;
  }

  /* Include :focus-visible so keyboard users get the same reveal as pointer users. */
  .reveal:is(:hover, :focus-visible) {
    text-decoration-inset: 0 0;
  }
}

<a class="draw" href="#">Hover to draw underline</a>

.draw {
  position: relative;
  display: inline-block;
  color: inherit;
  text-decoration: none;
  padding-block-end: 0.4em; /* Space for the line below the text; example value */
}

.draw::after {
  content: "";
  position: absolute;
  inset: auto 0 0 0;
  block-size: 0.4em; /* Height of the squiggle; example value */
  pointer-events: none;
  border: var(--deco-thickness, max(1px, 0.08em)) solid var(--deco-color, currentColor);
  /* A single shape strokes along its path, using the border width as the stroke. Example squiggle. */
  border-shape: shape(from 0% 50%,
    curve to 20% 50% with 5% 0% / 15% 100%,
    smooth to 40% 50% with 35% 100%,
    smooth to 60% 50% with 55% 100%,
    smooth to 80% 50% with 75% 100%,
    smooth to 100% 50% with 95% 100%);
}

/* Only hide and animate the line when the user has not asked for reduced motion;
   otherwise it stays fully drawn. */
@media (prefers-reduced-motion: no-preference) {
  .draw::after {
    /* Clip the line away from the right edge. The negative block insets keep the stroke
       from being cut off vertically. */
    clip-path: inset(-1em 100% -1em 0);
    /* A slightly slower draw than a hover reveal reads as drawing; example duration. */
    transition: clip-path var(--draw-duration, 0.6s) ease-out;
  }

  .draw:is(:hover, :focus-visible)::after {
    clip-path: inset(-1em 0 -1em 0); /* Fully drawn */
  }
}

/* Gradient outline with a see-through middle */
.ring {
  padding: 0.2em 0.75em; /* Example values: enough room that the outline does not touch the text */
  /* The border width is the visible outline thickness. Relative to the font size, with a 2px
     minimum so it stays visible on small text; example value. */
  border: var(--ring-width, max(2px, 0.12em)) solid transparent;
  background: linear-gradient(135deg, var(--ring-start, #2563eb), var(--ring-end, #db2777)) border-box;
  /* Paint the background only under the border, so the inside stays transparent. */
  background-clip: border-area;
}

/* Border that follows a shape that border-radius cannot draw, such as a tag with a pointed end */
.tag {
  padding: 0.2em 1.5em 0.2em 0.9em; /* Example values: extra room at the point so the text is not clipped */
  border: var(--shape-stroke, max(2px, 0.08em)) solid var(--shape-color, currentColor); /* Example stroke width */
  /* A single shape strokes along its path. Two shapes fill the area between them.
     This polygon is an example; use shape() for curves. */
  border-shape: polygon(0 0, 85% 0, 100% 50%, 85% 100%, 0 100%);
}

.label {
  padding: 0.2em 0.9em; /* Example values */
  border: var(--shape-stroke, max(2px, 0.08em)) solid var(--shape-color, currentColor);
  border-radius: 0.6em; /* Corner size; example value */
  corner-shape: bevel; /* round (default) | scoop | bevel | notch | square | squircle */
}

.ring {
  border: var(--ring-width, max(2px, 0.12em)) solid transparent; /* Example width */
  /* Fallback: an opaque surface layer hides the gradient inside the border. */
  background:
    linear-gradient(var(--surface, canvas) 0 100%) padding-box,
    linear-gradient(135deg, var(--ring-start, #2563eb), var(--ring-end, #db2777)) border-box;
}

@supports (background-clip: border-area) {
  .ring {
    background: linear-gradient(135deg, var(--ring-start, #2563eb), var(--ring-end, #db2777)) border-box;
    background-clip: border-area; /* The inside stays transparent */
  }
}

@supports not (background-clip: border-area) {
  .ring {
    position: relative;
    border-color: transparent;
  }

  .ring::before {
    content: "";
    position: absolute;
    inset: calc(var(--ring-width, max(2px, 0.12em)) * -1); /* Cover the border box */
    padding: var(--ring-width, max(2px, 0.12em)); /* Ring thickness */
    border-radius: inherit;
    background: linear-gradient(135deg, var(--ring-start, #2563eb), var(--ring-end, #db2777));
    /* Keep only the area between the content box and the border box. */
    mask: linear-gradient(#000 0 0) content-box, linear-gradient(#000 0 0);
    mask-composite: exclude;
    pointer-events: none;
  }
}

.tag {
  --stroke: max(2px, 0.08em); /* Example stroke width */
  position: relative;
  padding: 0.2em 1.5em 0.2em 0.9em; /* Example values */
  border: var(--stroke) solid transparent;
}

@supports not (border-shape: circle(50%)) {
  .tag::before {
    content: "";
    position: absolute;
    inset: calc(var(--stroke) * -1); /* Cover the border box */
    background: var(--shape-color, currentColor);
    pointer-events: none;
    /* Outer shape, then the inner shape inset by the stroke. The inner points are approximate on
       diagonal edges, so the stroke looks slightly thinner there. */
    clip-path: polygon(evenodd,
      0 0, 85% 0, 100% 50%, 85% 100%, 0 100%,
      var(--stroke) var(--stroke),
      calc(85% - var(--stroke)) var(--stroke),
      calc(100% - var(--stroke) * 2) 50%,
      calc(85% - var(--stroke)) calc(100% - var(--stroke)),
      var(--stroke) calc(100% - var(--stroke)));
  }
}

@supports (border-shape: circle(50%)) {
  .tag {
    border-color: var(--shape-color, currentColor);
    border-shape: polygon(0 0, 85% 0, 100% 50%, 85% 100%, 0 100%);
  }
}