这是一个非常好的问题。在我所看到的任何地方，CSS文件都倾向于在一段时间后失去控制——尤其是，但不仅仅是在团队中工作时。
以下是我自己努力遵守的规则(并不是说我总能做到)。
Refactor early, refactor often. Frequently clean up CSS files, fuse together multiple definitions of the same class. Remove obsolete definitions immediately.
When adding CSS during fixing bugs, leave a comment as to what the change does ("This is to make sure the box is left aligned in IE < 7")
Avoid redundancies, e.g. defining the same thing in .classname and .classname:hover.
Use comments /** Head **/ to build a clear structure.
Use a prettifier tool that helps maintain a constant style. I use Polystyle, with which I'm quite happy (costs $15 but is money well spent). There are free ones around as well (e.g. Code Beautifier based on CSS Tidy, an open-source tool).
Build sensible classes. See below for a few notes on this.
Use semantics, avoid DIV soup - use <ul>s for menus, for example.
Define everything on as low a level as possible (e.g. a default font family, colour and size in the body) and use inherit where possible
If you have very complex CSS, maybe a CSS pre-compiler helps. I'm planning to look into xCSS for the very same reason soon. There are several others around.
If working in a team, highlight the necessity of quality and standards for CSS files as well. Everybody's big on coding standards in their programming language(s), but there is little awareness that this is necessary for CSS too.
If working in a team, do consider using Version Control. It makes things that much easier to track, and editing conflicts that much easier to solve. It's really worth it, even if you're "just" into HTML and CSS.
Do not work with !important. Not only because IE =< 7 can't deal with it. In a complex structure, the use of !important is often tempting to change a behaviour whose source can't be found, but it's poison for long-term maintenance.
构建合理的类
这就是我喜欢构建合理类的方式。
我首先应用全局设置:
body { font-family: .... font-size ... color ... }
a { text-decoration: none; }
然后，我确定页面布局的主要部分。顶部区域、菜单、内容和页脚。如果我写了好的标记，这些区域将与HTML结构相同。
然后，我开始构建CSS类，在合理的情况下指定尽可能多的祖先，并将相关类尽可能紧密地分组。
div.content ul.table_of_contents
div.content ul.table_of_contents li
div.content ul.table_of_contents li h1
div.content ul.table_of_contents li h2
div.content ul.table_of_contents li span.pagenumber
你可以把整个CSS结构想象成一棵树，它的定义越远越具体。你想要尽可能地减少课程的数量，并且尽可能少地重复学习。
例如，假设您有三个级别的导航菜单。
这三个菜单看起来不同，但它们也有一些共同的特征。例如，它们都是<ul>，它们都有相同的字体大小，并且项目都是彼此相邻的(与ul的默认呈现相反)。此外，所有菜单都没有项目符号(list-style-type)。
首先，在一个名为menu的类中定义公共特征:
div.navi ul.menu { display: ...; list-style-type: none; list-style-image: none; }
div.navi ul.menu li { float: left }
然后，定义这三个菜单的具体特征。第1级是40像素高;2级和3级，20像素。
注意:您也可以为此使用多个类，但Internet Explorer 6在使用多个类时存在问题，因此本例使用id。
div.navi ul.menu#level1 { height: 40px; }
div.navi ul.menu#level2 { height: 20px; }
div.navi ul.menu#level3 { height: 16px; }
菜单的标记看起来像这样:
<ul id="level1" class="menu"><li> ...... </li></ul>
<ul id="level2" class="menu"><li> ...... </li></ul>
<ul id="level3" class="menu"><li> ...... </li></ul>
如果页面上有语义相似的元素(比如这三个菜单)，首先试着找出共性，然后把它们放到一个类中;然后，计算出特定的属性并将它们应用到类中，或者，如果您必须支持Internet Explorer 6，则应用ID。
其他HTML技巧
如果你将这些语义添加到HTML输出中，设计师以后可以使用纯CSS定制网站和/或应用程序的外观，这是一个很大的优势和节省时间。
If possible, give every page's body a unique class: <body class='contactpage'> this makes it very easy to add page-specific tweaks to the style sheet:
body.contactpage div.container ul.mainmenu li { color: green }
When building menus automatically, add as much CSS context as possible to allow extensive styling later. For example:
<ul class="mainmenu">
<li class="item_first item_active item_1"> First item </li>
<li class="item_2"> Second item </li>
<li class="item_3"> Third item </li>
<li class="item_last item_4"> Fourth item </li>
</ul>
This way, every menu item can be accessed for styling according to its semantic context: Whether it's the first or last item in the list; Whether it's the currently active item; and by number.
请注意，如上例中所述的多个类的分配在IE6中不能正常工作。有一个变通方案可以使IE6能够处理多个类。如果没有解决方法，则必须设置对您来说最重要的类(项目编号、活动或第一个/最后一个)，或者使用id。
