Alignment with relative and absolute positioning - html

Question

How could I center the blue box inside the red one ?
I see that the left side of the blue box is exactly in the middle of the red box, but I would like to center the whole blue box, not its left side. The dimensions of the boxes are not constant. I want to align regardless of boxes dimensions. Example to play with here. Thanks !
HTML:
<div id="rel">
<span id="abs">Why I'm not centered ?</span>
</div>
CSS:
#rel {
position: relative;
top: 10px;
left: 20px;
width: 400px;
height: 300px;
border: 1px solid red;
text-align: center;
}
#abs {
position: absolute;
bottom: 15px;
width: 300px;
height: 200px;
border: 1px solid blue;
}

Answer 1

If you're able to change the <span> tag to a <div>
<div id="rel">
<div id="abs">Why I'm not centered ?</div>
</div>
Then this piece of CSS should work.
#rel {
position: absolute;
top: 10px;
left: 20px;
width: 400px;
height: 300px;
border: 1px solid red;
text-align: center; }
#abs {
width: 300px;
height: 200px;
border: 1px solid blue;
margin: auto;
margin-top: 50px; }
I think it's better to use more automation for the enclosed box as less changes would be needed should you change the size of the container box.

Answer 2

You could add left:50px to #abs if that's all you want...
#abs {
position: absolute;
bottom: 15px;
width: 300px;
height: 200px;
border: 1px solid blue;
left:50px;
}

Answer 3

If you are going to define dimensions like that (200px x 300px and 300px x 400px), here's how it can be centered:
#rel {
position: relative;
top: 10px;
left: 20px;
width: 400px;
height: 300px;
border: 1px solid red;
text-align: center;
}
#abs {
position: absolute;
width: 300px;
height: 200px;
border: 1px solid blue;
margin: 49px 0 0 49px;
}

Answer 4

You can check at my solution here at http://jsfiddle.net/NN68Z/96/
I did the following to the css
#rel {
position: relative;
top: 10px;
left: 20px;
right: 20px;
width: 400px;
height: 300px;
border: 1px solid red;
text-align: center;
display: table-cell;
vertical-align: middle;
}
#abs {
display: block;
bottom: 15px;
width: 300px;
height: 200px;
border: 1px solid blue;
margin: 0 auto;
}

Answer 5

This should work
#abs {
position: absolute;
left: auto;
right: auto;
bottom: 15px;
width: 300px;
height: 200px;
border: 1px solid blue;
}