!important within keyframes declarations is completely ignored in some browsers and therefore it should not be used to be consistent among all browsers.

Noncompliant Code Example

@keyframes kf {
  from { margin-top: 50px; }
  50%  { margin-top: 150px !important; } /* Noncompliant; ignored */
  to   { margin-top: 100px; }
}

Compliant Solution

@keyframes kf {
  from { margin-top: 50px; }
  50%  { margin-top: 150px; }
  to   { margin-top: 100px; }
}

See