Why is this an issue?

!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; }
}

Resources