Why centering with margin 0 auto works with display:block but does not work with display:inline-block ?

Question

Just a quick question that was bugging my mind : Why centering with

margin:0 auto

does work fine with

display:block

but does not center a div when display is set to

display:inline-block

Thanks for answers

Answer 1

My understanding is as follows (though I am happy to be corrected).

• Inline elements do not have a width property, and so the "auto" cannot be calculated.
• Block elements have a width property, so the width of the "auto" can be calculated.
• Inline-block elements have an outside which acts inline, but an inside which acts like a block. As such, the width set acts more like the width of a word in an inline element.

Answer 2

See http://www.w3.org/TR/CSS2/visudet.html#Computing_widths_and_margins

Blocks:

10.3.3 Block-level, non-replaced elements in normal flow

The following constraints must hold among the used values of the other properties:

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

If 'width' is not 'auto' and 'border-left-width' + 'padding-left' + 'width' + 'padding-right' + 'border-right-width' (plus any of 'margin-left' or 'margin-right' that are not 'auto') is larger than the width of the containing block, then any 'auto' values for 'margin-left' or 'margin-right' are, for the following rules, treated as zero.

If all of the above have a computed value other than 'auto', the values are said to be "over-constrained" and one of the used values will have to be different from its computed value. If the 'direction' property of the containing block has the value 'ltr', the specified value of 'margin-right' is ignored and the value is calculated so as to make the equality true. If the value of 'direction' is 'rtl', this happens to 'margin-left' instead.

If there is exactly one value specified as 'auto', its used value follows from the equality.

If 'width' is set to 'auto', any other 'auto' values become '0' and 'width' follows from the resulting equality.

If both 'margin-left' and 'margin-right' are 'auto', their used values are equal. This horizontally centers the element with respect to the edges of the containing block.

Inline-blocks:

10.3.9 'Inline-block', non-replaced elements in normal flow

If 'width' is 'auto', the used value is the shrink-to-fit width as for floating elements.

A computed value of 'auto' for 'margin-left' or 'margin-right' becomes a used value of '0'.

Answer 3

I realise that I am a bit late answering this, but you can get around this issue if you want to combine margin auto with inline-block by specifying text align in body as in ...

body {
    text-align: center;
}

Answer 4

inline-block elements don't respect right or left margin, therefor the setting auto can't be applied.

block elements respect all margins.

Answer 5

You need to learn about inline boxes (not blocks) to see what is happening. An inline box contains inline items such as text and images. An inline block maintains its position within the inline box but acts as a block element within its position inside the box. Just like a word in the text cannot have its own width, neither can an inline block.

The actual spec for line boxes.