BEM 避免 选择器嵌套， 不然不可复用

目的让 the logo and the authorization form 互相交换位置而不通过css 和js
1. Block: 通过class 设置块，是有意义的
不能影响它的环境，不能设置它的形状 边距等
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Block nesting 嵌套
可以嵌套 且可以嵌套多层
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2.Element

A composite part of a block that can’t be used separately from it.
Features:

The element name describes its purpose (“What is this?” — item, text, etc.), not its state (“What type, or what does it look like?” — red, big, etc.).

The structure of an element’s full name is block-name__element-name. The element name is separated from the block name with a double underscore (__).

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element nesting 元素嵌套
An element is always part of a block, not another element. This means that element names can’t define a hierarchy such as block__elem1__elem2.
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块是定义一个作用域， 确保所有元素element 在block的前缀之下, block__element 一定在 block 包围里

Not all blocks have elements. An element is an optional block component. 元素是个可选择的块
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Should I create a block or an element?
Create a block
If a section of code might be reused and it doesn’t depend on other page components being implemented.

Create an element
If a section of code can’t be used separately without the parent entity (the block).
The exception is elements that must be divided into smaller parts – subelements – in order to simplify development. In the BEM methodology, you can’t create elements of elements. In a case like this, instead of creating an element, you need to create a service block.

3.Modifier

The same block looks different due to the use of a modifier.
An entity that defines the appearance, state, or behavior of a block or element.
Features:

The modifier name describes its appearance (“What size?” or “Which theme?” and so on — size_s or theme_islands), its state (“How is it different from the others?” — disabled, focused, etc.) and its
以 _ 做分离
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Key-value
Used when the modifier value is important. For example, “a menu with the islands design theme”: menu_theme_islands.

The structure of the modifier’s full name follows the pattern:

block-name_modifier-name_modifier-value

block-name__element-name_modifier-name_modifier-value
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A modifier can’t be used alone，
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incorrent

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BEM entity
Blocks, elements, and modifiers are all called BEM entities.

Mix
A technique for using different BEM entities on a single DOM node.

Mixes allow you to:
Combine the behavior and styles of multiple entities without duplicating code.
Create semantically new UI components based on existing ones.
Example
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File structure
[cc]search-form/ # Directory of the search-form

__input/ # Subdirectory of the search-form__input
search-form__input.css # CSS implementation of the
# search-form__input element
search-form__input.js # JavaScript implementation of the
# search-form__input element

__button/ # Subdirectory of the search-form__button
# element
search-form__button.css
search-form__button.js

_theme/ # Subdirectory of the search-form_theme
# modifier
search-form_theme_islands.css # CSS implementation of the search-form block
# that has the theme modifier with the value
# islands
search-form_theme_lite.css # CSS implementation of the search-form block
# that has the theme modifier with the value
# lite

search-form.css # CSS implementation of the search-form block
search-form.js # JavaScript implementation of the
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BEM tree
Let’s consider an example of a DOM tree:
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• en
• ru

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BEM xml

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HTML implementation:
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//Positioning a block relative to other blocks

CSS implementation:

.page__header {
padding: 20px;
}

.page__footer {
padding: 50px;
}
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Positioning elements inside a block
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CSS implementation:

.page__inner {
margin-right: auto;
margin-left: auto;
width: 960px;
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Naming 命名
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.button__icon {}
.button__text {}
.button_theme_islands {}

…

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Using HTML wrappers 使用包装
[cc lang=”html”]Positioning a block relative to other blocks

Positioning HTML elements inside a block





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