Is it safe to remove a property from the `0%` keyframe of a CSS animation if it is already declared in the class the animation is applied to?

Question

Context:

According to the 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.

This can lead to two different interpretations:

• A) Declare the property in the class and not in the 0% or from keyframe.

• B) Declare the property in the class and in the 0% or from keyframe.

---

Simplified Example:

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>

---

While both have the same end result, following the Don't Repeat Yourself (DRY) principle, A) could vastly reduce code for all the animations that use more than one property.

---

Complex Example:

/*
   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>

---

Question:

Is it safe to remove a property from the 0% keyframe of a CSS animation if it is already declared in the class the animation is applied to?

Is doing this:

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

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

considered safe across browsers? Or is it to be expected that user agents render different results or performance?

---

TESTS:

Rendering the same result:

Windows 10 / 64-bit

• Chrome 54.0.2840.71 m (64-bit)
• FireFox 49.0.2
• Edge 25.10586.0.0

Answer 1

The computed value of a property can vary depending on what CSS rules are applying to a given element, the specificity of these rules, whether the element has an inline style declaration for that property, whether a script is modifying the value of that property at runtime, etc.

If you want an animation to start from a predictable, and fixed, value, you will need to specify this value in the 0% keyframe so that it doesn't start animating from what the computed value happens to be at the time the animation starts instead.

You can leave out the 0% keyframe if you can guarantee that the computed value for that element at the time the animation is started is always a constant value. You must leave out the 0% keyframe if you want the animation to always start from whatever the computed value happens to be at the time, if it can change.

Similar rules apply to the 100% keyframe: if the animation needs to end with a fixed value regardless of what the computed value may be, you will need to specify the 100% keyframe; otherwise, if it needs to end with the same value as the computed value, you will need to leave it out.

Answer 2

It can work in both way, as the specification point out.

Regarding safeness, this should be ok in the following browsers:

IE 11
Edge 14
Firefox from 47 to 52
Chrome  from 49 to 57
Safari  from 9.1 to 10

Keeping the 0% in my opinion could make your code more readable specially if you have complex animation, but in case you want avoid repetition you could use some CSS pre-processor, for example LESS using animation keyframes mixin or other techniques.