why bootstrap uses floats on .span instead of display: inline-block?

I prefer grid systems that use display: inline-block; rather than float, such as Pure (formerly Yahoo YUI Grid), because they internationalise without extra styles; change the text direction to right-to-left and the layout automatically reverses; floats don't do this. Floats also introduce the need to clear and other cross browser oddities. Any inaccuracies that inline-block may have over float can be remedied, as demonstrated by Pure. As to the criticism that display: inline-block; is not meant to be used for layout, perhaps the use of display: table; should also be banned for cross-browser centring. I would also question whether the term Semantic Web really applies to CSS since the term is primarily concerned with HTML and using its elements and attributes to impart machine readable meaning; the whole point of CSS is to style semantic HTML as radically as one wants to, hence classic sites like CSS Zen Garden.

I say that as long as the technique is not exploiting a bug, is not obstructing content to users and devices and is sufficiently supported then it is acceptable. There is no reason that one can't use CSS in unorthodox, but supported, ways, just like Stu Nicholls' CSSPlay.

Interestingly, Flexible Box is also incorporated into Pure grids, a superior layout system compatible with modern browsers (≥IE10 and equivalent browsers).

Inline blocks are white-space aware, have an auto width of the actual content and stacks in the order they have in HTML. Floats aren't but require a clearfix method and are based on block elements. These elements have auto width on the available space horizontally. Purely semantically, inline-blocks are less semantic since the white-space aware format and importance of order. But looking at it purely in a functional way, both just aren't made for a grid. And if it wasn't for pseudo CSS, we would have un-semantic HTML tag clearfixe's as well. So in my believes they are both not winners. But there is no alternative for the time being when flexbox will become mandatory in upcoming years.

With inline blocks:

<div>
    <div style="display:inline-block; width:30%;">col1
    </div><div style="display:inline-block; width:70%;">col2</div>
</div>

Tags must be glued together/appended, to dismiss any gutter. The container div is necessary to force the items being part of a separate row.

With floats:

<div class="clearfix">
    <div style="float:left; width:30%;">col1</div>
    <div style="float:left; width:70%;">col2</div>
</div>

Clearfix is necessary to force a "row" (dismiss any normal flow items issues or floats after the floats)

Whether use one or the other is a matter of your goal (and taste). I must say I like inline-blocks more than floats, as long as you know the both of the col widths or use relative sizing (%). I thinks it's more intuitive and predictable than floats with clearfix, a fix" for issues that aren't even issues if it was used by how it should be used. Only the white-space awareness of inline blocks force you to use some funky html, that's a downside.

Ironically, tables do exactly all this (and even col-heights and vertically alignement's) without any issues. Since inline-blocks have to be placed in order, there's an motive for discussing here.

If we are talking about responsive, table 'loses' of inline-blocks. Let say you have 4 cols on a desktop and you want 2 cols on a tablet and 1 on a mobile. With inline blocks, you 'just' give the cols other width dimensions and they hopefully wrap nice (be aware of margins as they collapse). With tables, you're bound to actual rows, which can be quite stubborn. Flexbox is needed for a long time and looks beautiful. You can adjust the layout flexible on certain circumstances.

Bootstrap can be handy to learn how they did something. Just read in 3.0 they are using relative grid sizing. Which gives an issue with nested grids and aligning.

----  --a-  ----  ----
---b------
....  ..c.

Col a is a normal parent col. Col c is a child nested col of b. It's hard to align c with a with relative sizing since the gutter is variable to the container, unless you're using padding and border-box model. But that way you lose a lot of flexibility. When you want the col to have some background and padding, you're messing up the grid system, so you have to use container which you style, which would clutter the code. I haven't studied if they found a solution to this. I haven't yet. I went to fixed pixels, but that means in responsive design you can only have a few fixed width's and for everything around mobile use a relative grid.