根据Animation Spec：

If a ‘0%’ or ‘from’ keyframe is not specified,then the user agent
constructs a ‘0%’ keyframe using the computed values of the properties
being animated. If a ‘100%’ or ‘to’ keyframe is not specified,then
the user agent constructs a ‘100%’ keyframe using the computed values
of the properties being animated.

这可能导致两种不同的解释：

> A)在类中声明属性,而不是在0％或关键帧中声明属性.
> B)声明类中的属性以及0％或关键帧中的属性.

简化示例：

p:first-of-type {
  opacity: 0;
  animation: a 3s linear infinite alternate;
}
@keyframes a {
  100% {
    opacity: 1;
  }
}
p:last-of-type {
  opacity: 0;
  animation: b 3s linear infinite alternate;
}
@keyframes b {
  0% {
    opacity: 0;
  }
  100% {
    opacity: 1;
  }
}

<p>
  A) Declare the property in the class <strong>and not</strong> in the `0%` or `from` keyframe.
</p>
<p>
  B) Declare the property in the class <strong>and </strong> in the `0%` or `from` keyframe.
</p>

虽然两者具有相同的最终结果,但遵循Don’t Repeat Yourself (DRY)原则,A)可以大大减少使用多个属性的所有动画的代码.

复杂的例子：

/*
   Layout
*/

* {
  Box-sizing: border-Box;
}
body {
  margin: 0;
}
ul {
  margin: 0;
  padding: 0;
  list-style: none;
}
.container {
  height: 100vh;
  display: flex;
  flex-direction: column;
  justify-content: space-around;
  counter-reset: list-item;
}
.container > li {
  position: relative;
  counter-increment: list-item;
}
.container > li::before {
  content: counter(list-item,upper-alpha);
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  width: 100%;
  height: 100%;
  display: flex;
  justify-content: flex-end;
  align-items: flex-end;
  font-size: 1.5em;
  background-color: moccasin;
}
.container > li::after {
  content: '';
  position: absolute;
  top: 50%;
  left: 0;
  width: 100%;
  transform: translateY(-50%);
  height: 2px;
  background-color: gold;
}
/*
  SVG
*/

.svg-spritesheet {
  display: none;
}
.svg__icon {
  display: inline-block;
  vertical-align: middle;
  width: 1em;
  height: 1em;
}
.svg__icon--square {
  font-size: 5em;
  color: dodgerblue;
}
/*
  Question Related
*/

.container > li:first-of-type .svg__icon--square {
  opacity: 0;
  transform: scale(.5) rotate(45deg);
  animation: animationA 5s linear infinite alternate;
}
.container > li:nth-child(2) .svg__icon--square {
  opacity: 0;
  transform: scale(.5) rotate(45deg);
  animation: animationB 5s linear infinite alternate;
}
@keyframes animationA {
  to {
    opacity: 1;
    transform: translateX(500px) scale(1) rotate(90deg);
  }
}
@keyframes animationB {
  from {
    opacity: 0;
    transform: scale(.5) rotate(45deg);
  }
  to {
    opacity: 1;
    transform: translateX(500px) scale(1) rotate(90deg);
  }
}

<ul class="container">
  <li>
    <svg class="svg__icon svg__icon--square">
      <use xlink:href="#svg-icon-square"></use>
    </svg>
  </li>
  <li>
    <svg class="svg__icon svg__icon--square">
      <use xlink:href="#svg-icon-square"></use>
    </svg>
  </li>
</ul>

<svg class="svg-spritesheet">
  <symbol id="svg-icon-square" viewBox="0 0 32 32">
    <title>Demo Square</title>
    <rect width="32" height="32" fill="currentColor" />
  </symbol>
</svg>

题：

如果已经在应用动画的类中声明了CSS动画的0％关键帧中的属性,是否安全？

这样做：

.el {
  opacity: 0;
  animation: example 1s;
}

@keyframes example {
  100% {
    opacity: 1;
  }
}

跨浏览器认为安全？或者是否期望用户代理呈现不同的结果或性能？

测试：

渲染相同的结果：

Windows 10/64位

> Chrome 54.0.2840.71米(64位)
> FireFox 49.0.2
> Edge 25.10586.0.0