使用box-shadow制作一些简单的鼠标悬停效果

            
            /*
  https://developer.mozilla.org/en/docs/Web/CSS/box-shadow
  box-shadow: [inset?] [top] [left] [blur] [size] [color];

  提示：
    把所有的模糊效果都设置为0，就能一个纯色的填充效果
    加上inset关键字，这样box-shadow就会在元素的内部显示。
    当你把鼠标悬停在上面时，动画效果会让元素从你指定的那一边开始填充（比如top和left接受负值来变成bottom和right）。
    你可以堆叠多个阴影效果。
    如果你正在动画多个阴影，记得在悬停/聚焦时保持与非悬停/聚焦时相同的阴影数量（即使你需要创建一个透明的阴影），这样动画才会更流畅。否则，动画就会显得断断续续的。
*/

// Animate the size, inside
.fill:hover,
.fill:focus {
  box-shadow: inset 0 0 0 2em var(--hover);
}

// Animate the size, outside
.pulse:hover, 
.pulse:focus {
  animation: pulse 1s;
  box-shadow: 0 0 0 2em transparent;
}

@keyframes pulse {
  0% { box-shadow: 0 0 0 0 var(--hover); }
}

// Stack multiple shadows, one from the left, the other from the right
.close:hover,
.close:focus {
  box-shadow: 
    inset -3.5em 0 0 0 var(--hover),
    inset 3.5em 0 0 0 var(--hover);  
}

// Size can also be negative; see how it's smaller than the element
.raise:hover,
.raise:focus {
  box-shadow: 0 0.5em 0.5em -0.4em var(--hover);
  transform: translateY(-0.25em);
}

// Animating from the bottom
.up:hover,
.up:focus {
  box-shadow: inset 0 -3.25em 0 0 var(--hover);
}

// And from the left
.slide:hover,
.slide:focus {
  box-shadow: inset 8.5em 0 0 0 var(--hover);
}

// Multiple shadows, one on the outside, another on the inside
.offset {  
  box-shadow: 
    0.3em 0.3em 0 0 var(--color),
    inset 0.3em 0.3em 0 0 var(--color);
  
  &:hover,
  &:focus {
    box-shadow: 
      0 0 0 0 var(--hover),
      inset 6em 3.5em 0 0 var(--hover);
  }
}

//=== Set button colors
// If you wonder why use Sass vars or CSS custom properties...
  // Make a map with the class names and matching colors
$colors: (
  fill: #a972cb,
  pulse: #ef6eae, 
  close: #ff7f82, 
  raise: #ffa260, 
  up: #e4cb58, 
  slide: #8fc866, 
  offset: #19bc8b
);

// Sass variables compile to a static string; CSS variables are dynamic and inherited
  // Loop through the map and set CSS custom properties using Sass variables
@each $button, $color in $colors {
  .#{$button} {
    --color: #{$color};
    --hover: #{adjust-hue($color, 45deg)};
  }
}

// Now every button will have different colors as set above. We get to use the same structure, only changing the custom properties.
button {  
  color: var(--color);
  transition: 0.25s;
  
  &:hover,
  &:focus { 
    border-color: var(--hover);
    color: #fff;
  }
}

//=== Pen styling, ignore
body {
  color: #fff;
  background: hsl(227, 10%, 10%);
  font: 300 1em 'Fira Sans', sans-serif;
  
  // Center everything ever
  justify-content: center;
  align-content: center;
  align-items: center;
  text-align: center;
  min-height: 100vh;
  display: flex;
}

// Basic button styles
button {
  background: none;
  border: 2px solid;
  font: inherit;
  line-height: 1;
  margin: 0.5em;
  padding: 1em 2em;
}

h1 { font-weight: 400; }

code { 
  color: #e4cb58;
  font: inherit;
}