css absolute position won't work with margin-l

Question

Say you have the following css applied to a div tag

.divtagABS {
    position: absolute;
    margin-left: auto;  
    margin-right:auto;
 }

the margin-left and margin-right does not take effect

but if you have relative, it works fine i.e.

,divtagREL {
position: relative;
margin-left: auto;  
 margin-right:auto;
}

why is that? i just want to center an element

can someone explain why setting margins to auto in absolute position does not work?

Answer 1

I've used this trick to center an absolutely positioned element. Though, you have to know the element's width.

.divtagABS {
    width: 100px;
    position: absolute;
    left: 50%;
    margin-left: -50px;
  }

Basically, you use left: 50%, then back it out half of it's width with a negative margin.