px fallback for rem units should not be removed in compatibility mode

[ademarre]

When using rem, a common way to add support for IE8 is with a px fallback:

.example {
  font-size: 24px;    /* Fallback for IE8 */
  font-size: 1.5rem;
  padding: 32px 16px; /* Fallback for IE8 */
  padding: 2rem 1rem;
}

The clean-css advanced optimizations are removing the fallback, even when compatibility is on. It would be great if compatibility mode could detect and preserve px fallbacks.

My current automated solution is to run robwierzbowski/node-pixrem after clean-css in my build. (See gruntjs/grunt-contrib-cssmin#112 (comment))
#186 addressed a related edge case.

[GoalSmashers]

@ademarre this is due to a fact not all properties have advanced optimisations implemented. This is due in version 2.4 with #290. So far please use your workaround or a --skip-aggressive-merging switch.

PS) #186 addressed basic optimisations not advanced ones so not such easy fix this time.