Valid keyframe lists

In order for a keyframe list to be valid, it must include rules for at least the times 0% (or from) and 100% (or to) (that is, the starting and ending states of the animation). If both of these time offsets aren't specified, the keyframe declaration is invalid and can't be used for animation.

If you include properties that can't be animated in your keyframe rules, they get ignored, but the supported properties will still be animated.

Duplicate resolution

If multiple keyframe sets exist for a given name, the last one encountered is used. @keyframes rules don't cascade, so animations never drive keyframes from more than one rule set.

If a given animation time offset is duplicated, the last keyframe in the @keyframes rule for that percentage is used for that frame. There's no cascading within a @keyframes rule if multiple keyframes specify the same percentage values.

When properties are left out of some keyframes

Any properties that you don't specify in every keyframe are interpolated (with the exception of those that can't be interpolated, which are instead dropped from the animation entirely). For example:

@keyframes identifier {
  0% { top: 0; left: 0; }
  30% { top: 50px; }
  68%, 72% { left: 50px; }
  100% { top: 100px; left: 100%; }
}

Here, the top property animates using the 0%, 30%, and 100% keyframes, and left animates using the 0%, 70%, and 100% keyframes.

Only properties that are specified in both the 0% and 100% keyframes will be animated; any property not included in both of those keyframes will retain their starting value for the duration of the animation sequence.

Values

<identifier>

A name identifying the keyframe list. This must match the identifier production in CSS syntax.

from

A starting offset of 0%.

to

An ending offset of 100%.

<percentage>

A percentage of the time through the animation sequence at which the specified keyframe should occur.