HTML/CSS: Why is body not stretching to its contents?
Take this trivial example and open it:
<html>
<body style="background-image:url(http://upload.wikimedia.org/wikipedia/commons/2/21/Mandel_zoom_00_mandelbrot_set.jpg);background-repeat: no-repeat; background-position: top center;">
<div style="width: 8000px; border: 3px solid red;">Demo</div>
</body>
</html>
The page is made so that the body has a top-centered background picture and a containing element which overflows window boundaries so there is horizontal scrolling (if you have a monitor wider than 8000px then you're really cool, please make the window smaller and refresh).
The problem is that for some reason the <body> doesn't stretch to contain the <div>. It just stays the same width as the viewport and the <div> overflows it. This in turn causes the background to be centered at the wrong place and crops it to the size of the viewport. Quite ugly when you scroll to the right.
I've already found a solution to this problem, but I'm wondering WHY this i开发者_如何学编程s so? It seems to be consistent across browsers too. But in my opinion this is quite counter-intuitive and basically plain wrong. The container element should be big enough to contain it's children - unless they are absolutely positioned of course in which case they don't participate in the layout calculations.
Blocks simply do not stretch horizontally to accomodate their child content, at all; never have.(1) That's something that only happens in the vertical axis.(2) Logically, both dimensions can't be stretchy; one has to be fixed (wrt parent). In CSS, the width of a block element in normal flow is derived solely from the parent (minus paddings, margins, borders), and then the height follows on from that, seeing how much content you can fit in the block's width and flowing it.
The background image can appear in a confusing place that misleads you to what is actually happening because of this quirk of CSS:
For documents whose root element is an HTML "HTML" element or an XHTML "html" element that has computed values of 'transparent' for 'background-color' and 'none' for 'background-image', user agents must instead use the computed value of the background properties from that element's first HTML "BODY" element or XHTML "body" element child when painting backgrounds for the canvas, and must not paint a background for that child element. Such backgrounds must also be anchored at the same point as they would be if they were painted only for the root element.
This is a nasty hack put in place because people were used to putting backgrounds on ‘body’ and having it fill the viewport, even though the <body> element itself does not represent the viewport.
(1) except in a few special cases, like float and position: absolute elements without a declared width.
(2) unless you deliberately stop that by setting height/overflow.
The exception to expanding is the viewport. Nothing expands the viewport unless its direct or attached contents require it -- this includes that div and table elements (eg, display: table-cell on body) -- but not a block container.
加载中,请稍侯......
精彩评论