BEM规范

• 仅从名字就能知道一个 CSS 选择器具体做什么
• 从名字能大致清楚一个选择器可以在哪里使用
• 从 CSS 类的名称可以看出它们之间的联系

B代表区块(Block)

在实际中，这里『区块』可以表示一个网站导航、页眉、页脚或者其他一些设计区块。

Encapsulates a standalone entity that is meaningful on its own. While blocks can be nested and interact with each other, semantically they remain equal; there is no precedence or hierarchy. Holistic entities without DOM representation (such as controllers or models) can be blocks as well.

Naming

Block names may consist of Latin letters, digits, and dashes. To form a CSS class, add a short prefix for namespacing: .block

html

Any DOM node can be a block if it accepts a class name.

 <div class="block">...</div>

css

• Use class name selector only
• No tag name or ids
• No dependency on other blocks/elements on a page

 .block { color: #042; }

E代表元素(Elements)

Parts of a block and have no standalone meaning. Any element is semantically tied to its block.

Naming

Element names may consist of Latin letters, digits, dashes and underscores. CSS class is formed as block name plus two underscores plus element name: .block__elem

HTML

Any DOM node within a block can be an element. Within a given block, all elements are semantically equal.

 <div class="block">
       ...
       <span class="block__elem"></span>
     </div>

CSS

• Use class name selector only
• No tag name or ids
• No dependency on other blocks/elements on a page

Good

 .block__elem { color: #042; }

Bad

 .block .block__elem { color: #042; }
     div.block__elem { color: #042; }

M代表修饰符(Modifiers)

Flags on blocks or elements. Use them to change appearance, behavior or state.

Naming

Modifier names may consist of Latin letters, digits, dashes and underscores. CSS class is formed as block’s or element’s name plus two dashes: .block--mod or .block__elem--mod and .block--color-black with .block--color-red. Spaces in complicated modifiers are replaced by dash.

HTML

Modifier is an extra class name which you add to a block/element DOM node. Add modifier classes only to blocks/elements they modify, and keep the original class:

Good

 <div class="block block--mod">...</div>
     <div class="block block--size-big
         block--shadow-yes">...</div>

Bad

 <div class="block--mod">...</div>

CSS

Use modifier class name as selector:

 .block--hidden { }

To alter elements based on a block-level modifier:

 .block--mod .block__elem { }

Element modifier:

 .block__elem--mod { }

Example

Suppose you have block form with modifiers theme: "xmas" and simple: true and with elements input and submit, and element submit with its own modifier disabled: true for not submitting form while it is not filled:

HTML

/səbˈmɪt/

 <form class="form form--theme-xmas form--simple">
   <input class="form__input" type="text" />
   <input
     class="form__submit form__submit--disabled"
     type="submit" />
 </form>

CSS

.form { }
 .form--theme-xmas { }
 .form--simple { }
 .form__input { }
 .form__submit { }
 .form__submit--disabled { }