For example, consider this CSS file:

input:-ms-input-placeholder {
  font-size: 14px;
}
input::-ms-input-placeholder {
  font-size: 14px;
}

(Note: different numbers of colons)

If you run cssnano against this file with the default preset, you get:

input:-ms-input-placeholder,input::-ms-input-placeholder{font-size:14px}

Unfortunately, this won't work with Internet Explorer. These rules cannot be safely merged. When I disable the mergeRules optimization, this issue is resolved.

This issue may be related to #297