calc function is a function introduced in CSS3, it can be used to calculate length values. There are a few good features which make it suitable for aligning an element vertically.One good part is that the operands can have different units like percentage, px, rem etc. This makes it very flexible when calculating the length value. One example:.rect{ margin-top:20px; height:50px; background:green; width:calc(100%-20px);}Here the left operand uses percentage while the right one uses px, they can work perfectly without any conflict.The result of calc is dynamic but not pre -calculated....