Свойства min-height, min-width, max-height, max-width должны иметь всегда фиксированное значение?

However, it’s acceptable to set min-height and height when the values are different units:

#example {
  min-height: 3em;
  height: 138px;
}

Here, the min-height declaration is based on em, which means that at some stage (due to text resizing) the em height may be larger than the 138px height we’ve set. In cases such as this, the element will be allowed to expand further than the 138px height, thus accommodating the resizing of the em-based text.