Confusion about auto margins with absolute positioning

user_19079176 0
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MDN suggests that margin-left: auto with position: absolute will set the left margin to 0, but that does not seem to be the case in practice. I am now wondering if it is a bug in the browser, or in the MDN documentation as my example seems to align to the right.

fixed or absolute
0, except if both margin-left and margin-right are set to auto.

This example has margin-left set to auto and margin-right unspecified (i.e. initial value of 0) and the left margin appears to be computed as taking the remaining space rather than 0 as suggested by the source above: https://jsfiddle.net/40txneL7

I have also found a snippet in the spec suggesting that the remaining space is divided between the auto margins, and not set to 0 if just one axis margin is set.

Is MDN documentation incorrect here, or am I missing something?

css-position css html
2 Answers

There are a lot of rules concerning the width and the placement of the position absolute that you can read about here: https://www.w3.org/TR/css-position-3/#abs-non-replaced-width

The constraint that determines the used values for these elements is:

left + margin-left + border-left-width + padding-left + width + padding-right + border-right-width + margin-right + right = width of containing block

We don't have padding and the following rule apply:

If none of the three is auto: ... If one of margin-left or margin-right is auto, solve the equation for that value. If the values are over-constrained, ignore the value for left

So you will have

0px(left) + margin-left + 200px(width) + 0px(margin-right not specfied) + 0px(right) = width of containing block

The containing block in your case is the viewport so we have 100% of screen width. If we solve the equation you will get

margin-left = viewport width - 200px

so not equal to 0. You can verify this by checking the Dev tool

enter image description here

We set 0 for margin for the other cases. If you continue reading the Spec you can find:

Otherwise, set auto values for margin-left and margin-right to 0, and pick one of the following six rules that apply.

user_86203330

The current position of your absolutely positioned element is not due to margin-left: auto as you assume but to right: 0 since it overrides the left: 0 property.

Just remove right: 0 and you will see that your element is positioned left ignoring the margin-property.

user_143351530
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