Why Does Width Apply to a Button with Display Inline

Why does width apply to a button with display inline?

As mentioned in the comments, I'm pretty sure this has to do with browser-specific rendering behavior as is so typical of form elements. What I believe is happening when you set display: inline on the button is... nothing. Effectively, it's the same as the typical browser default display: inline-block, on which the width property does apply.

Refer to section 10.2, which describes the width property itself. In particular it explains why exactly the width property does not apply to inline elements (or inline boxes):

This property does not apply to non-replaced inline elements. The content width of a non-replaced inline element's boxes is that of the rendered content within them (before any relative offset of children). Recall that inline boxes flow into line boxes. The width of line boxes is given by the their containing block, but may be shorted by the presence of floats.

In short, it's because the content of inline elements resides in line boxes. The width of a line box cannot be controlled directly; it is determined entirely by the containing block and any incidental floats. You can see an example of line box rendering in section 9.4.2, which describes inline formatting contexts.

If display: inline actually made a button render as an inline box, all its contents would spill over and it would no longer look, or function, like a button. It makes sense to want to prevent that from happening, and I think that's just what browsers do.

So what exactly do they do to prevent this? Is a button a replaced element? I can't say for sure. But note, in section 9.2.2, it says:

Inline-level boxes that are not inline boxes (such as replaced inline-level elements, inline-block elements, and inline-table elements) are called atomic inline-level boxes because they participate in their inline formatting context as a single opaque box.

Section 10 does not explicitly mention atomic inline-level boxes, but it does have sections for calculating dimensions for inline replaced elements, as well as inline-block elements whether replaced or non-replaced, all of which are considered atomic inlines as mentioned above. In all of these cases, the width property applies as normal if it's not auto.

So, while it's still debatable whether or not a button is a replaced element, it probably doesn't matter at all for the purposes of this question. But it is still some kind of atomic inline element, since it still participates in an inline formatting context. For what it's worth, though, it appears to shrink to fit its contents if you don't set a width, so its behavior is probably closer to that of an inline-block in that case. One could say then that the actual value of display becomes inline-block, although this is never reflected in the developer tools because the computed value does not change (again a side effect of browser-specific rendering behavior).

input , button are inline, still I can set width and height

Depending on the version of CSS you are looking at, they are either replaced inline elements or they are display: inline-block;. Either way, height and width do apply.

Why are inline-block divs inside button becoming centered?

That happens cause by default, all buttons has text-align: center, if you force the button to have text-align: left it will go all to left

It is to notice that the text-align property does influence the alignment of the inline-blocks.

Issue with setting width of span tag

It's because inputs have default browser values like writing-mode & appearance and span elements are not.

If you give css to span like

span {
  appearance: auto;
  -webkit-writing-mod: horizontal-tb;
  width: 10%;
}

it will be works without change display property.

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