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CSS 常见问题 - CSS（层叠样式表）编辑

为什么我有效的CSS没有正确的渲染？

浏览器使用DOCTYPE声明来选择是否使用更符合Web标准或兼容旧浏览器的bug的模式。在你的HTML的开始使用一个正确的和现代的DOCTYPE声明将改善浏览器标准执行。

现代浏览器主要有两种渲染模式：

怪异模式： 又称向后兼容模式,，允许将传统网页渲染为作者意图。旧浏览器使用的非标准渲染规则。不完整的、不正确的、缺少DOCTYPE声明或已知的DOCTYPE声明中普遍使用2001年以前的文件将在怪异模式中呈现。
标准模式：浏览器试图严格遵守W3C标准。新HTML网页有望被设计为符合标准的浏览器，这样做的结果就是，用现代DOCTYPE声明的页面将被用标准模式呈现。

基于Gecko的浏览器，有三分之一 Almost Standards Mode，只有一些小怪癖。

这是最常用的触发标准模式或准标准模式的DOCTYPE声明列表：

<!DOCTYPE html> /* 这一行是 HTML5 的 doctype 声明。，使用该声明会使现代浏览器使用
                   HTML5 解析器处理页面，这是推荐的 doctype 声明。*/

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

为什么我有效的css完全没有被渲染？

为了使浏览器渲染样式文件，CSS样式表必须用text/css的MIME。如果Web服务器不服务于这种类型，则CSS也不会被应用。

id和class有什么不同？

HTML元素可以拥有一个id/或class属性。 id属性为元素指定应用一个有效名称，只能有一个具有该名称的元素。class属性指定一个类名的元素，而这个名称可以被页面内的许多元素被使用。 CSS允许你可以对特定的id和/或类名的元素应用样式。

当你想给一个特定元素或块应用样式规则时就使用ID选择符。此样式将只用于与该特定id匹配的元素。

当你想要将样式规则应用于多个块和元素时，你应该使用class选择符。

较少样式的样式表通常性能更高。因此建议尽可能多地使用类，保留id作为特定用途 (比如链接label标签和form元素或者为语义上唯一的元素应用样式)。

查看 CSS selectors

我如何还原属性的默认值？

最初CSS没有提供“defaule”关键字和还原默认属性的值，唯一途径是显式地重新声明该属性。

与CSS2相比，已经发生了改变。关键字 initial 现在是一个有效的CSS属性。它将给定的CSS属性值重置为默认值。

我如何才可以从一个样式中衍生出另一种样式?

CSS 不允许这样做。(See Eric Meyer's note about the Working Group's stance). 但是，将多个类分配给单个元素，可以提供相同的效果。

我该如何给一个元素分配多个类？

HTML元素可以通过列出的类属性，用空格分开它们。

<style type="text/css">
.news { background: black; color: white; }
.today { font-weight: bold; }
</style>

<div class="news today">
... content of today's news ...
</div>

如果相同的属性中声明的规则，解决冲突首先通过特异性，然后根据CSS声明的顺序。在class属性类的顺序是不相关的。

为什么我的样式规则不能正确地工作？

在语法上正确的样式规则可能在某些情况下不适用。你可以使用 DOM Inspector's CSS Style Rules 调试这类问题。下面列出了最常见的忽略样式规则的实例：

The way CSS styles are applied to HTML elements depends also on the elements hierarchy. It is important to remember that a rule applied to a descendent overrides the style of the parent, in spite of any specificity or priority of CSS rules.

.news { color: black; }
.corpName { font-weight: bold; color: red; }

<!-- news item text is black, but corporate name is red and in bold -->
<div class="news">
   (Reuters) <span class="corpName">General Electric</span> (GE.NYS) announced on Thursday...
</div>

In case of complex HTML hierarchies, if a rule seems to be ignored, check if the element is inside another element with a different style.

显式重定义样式规则

In CSS stylesheets, order is important. If you define a rule and then you re-define the same rule, the last definition is used.

#stockTicker { font-weight: bold; }
.stockSymbol { color: red; }
/*  other rules             */
/*  other rules             */
/*  other rules             */
.stockSymbol { font-weight: normal; }

<!-- most text is in bold, except "GE", which is red and not bold -->
<div id="stockTicker">
   NYS: <span class="stockSymbol">GE</span> +1.0 ...
</div>

To avoid this kind of error, try to define rules only once for a certain selector, and group all rules belonging to that selector.

使用便捷属性

Using shorthand properties for defining style rules is good because it uses a very compact syntax. Using shorthand with only some attributes is possible and correct, but it must be remembered that undeclared attributes are automatically reset to default. This means that a previous rule for a single attribute could be implicitly overridden.

#stockTicker { font-size: 12px; font-family: Verdana; font-weight: bold; }
.stockSymbol { font: 14px Arial; color: red; }

<div id="stockTicker">
   NYS: <span class="stockSymbol">GE</span> +1.0 ...
</div>

In the previous example the problem occurred on rules belonging to different elements, but it could happen also for the same element, because rule order is important.

#stockTicker {
   font-weight: bold;
   font: 12px Verdana;  /* font-weight is now normal */
}

使用 * 选择器

The * wildcard selector refers to any element, and it has to be used with particular care.

body * { font-weight: normal; }
#stockTicker { font: 12px Verdana; }
.corpName { font-weight: bold; }
.stockUp { color: red; }

<div id="section">
   NYS: <span class="corpName"><span class="stockUp">GE</span></span> +1.0 ...
</div>

In this example the body * selector applies the rule to all elements inside body, at any hierarchy level, including the .stockUp class. So font-weight: bold; applied to the .corpName class is overridden by font-weight: normal; applied to all elements in the body.

The use of the * selector should be minimized as it is a slow selector, especially when not used as the first element of a selector. Its use should be avoided as much as possible.

CSS 中的优先级

When multiples rules apply to a certain element, the rule chosen depends on its style specificity. Inline style (in HTML style attributes) comes first, followed by ID selectors, then class selectors and eventually element-name selectors.

div { color: black; }
#orange { color: orange; }
.green { color: green; }

<div id="orange" class="green" style="color: red;">This is red</div>

The rules are more complicated when the selector has multiple parts. More detailed information about how selector specificity is calculated can be found in the CSS 2.1 Specification chapter 6.4.3

-moz-， -ms-， -webkit-， -o- 以及 -khtml-* 属性有什么用？

These properties, called prefixed properties, are extension to the CSS standard. They are used to use experimental and non-standard features without polluting the regular namespace, preventing future incompatibilities to arise when the standard is extended.

The use of such properties on production websites is not recommended. If nevertheless needed, you are hinted to make a plan for the website evolution: these prefixed properties can be modified or even suppressed when the standard evolve.

Please see the Mozilla CSS Extensions page for more information on the Mozilla-prefixed CSS properties.