CSS absolute position won't work with margin-left:auto margin-right: auto

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?

EDIT : this answer used to claim that it isn't possible to center an absolutely positioned element with margin: auto;, but this simply isn't true. Because this is the most up-voted and accepted answer, I guessed I'd just change it to be correct.

When you apply the following CSS to an element

position: absolute;
top: 0;
left: 0;
right: 0;
bottom: 0;
margin: auto;

And then give the element a fixed width and height, such as 200px or 40%, the element will center itself.

Here's a Fiddle that demonstrates the effect.

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.

if the absolute element has a width,you can use the code below

.divtagABS{
    width:300px;
    positon:absolute;
    left:0;
    right:0;
    margin:0 auto;
}

Working JSFiddle below. When using position absolute, margin: 0 auto will not work, but you can do like this (will also scale):

left: 50%;
transform: translateX(-50%);

Update: Working JSFiddle

I already had this same issue and I've got the solution writing a container (.divtagABS-container, in your case) absolutely positioned and then relatively positioning the content inside it (.divtagABS, in your case).

Done! The margin-left and margin-right AUTO for your .divtagABS will now work.