BEM and managing CSS components

12/14/2015 By emehany

Large projects with a good amount of design and creative work require a strategy to organize and maintain all of its CSS components. In such large projects, it becomes harder to maintain or scale the components if there is no clear naming convention in place to identify the different components used in many different views. 

BEM ( Block, Element, Modifier )

 

The BEM approach ensures that everyone who participates in the development of a website works with a single codebase and speaks the same language. Using proper naming will prepare you for the changes in design of the website.

Block

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.

...
  • CSS
  • Use class name selector only
  • No tag name or id's
  • No dependency on other blocks/elements on a page
  • .block { color: #042; }

Element

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.

...
  • CSS
  • Use class name selector only
  • No tag name or id's
  • 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; }

 

Modifier

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__mod--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:

 

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 elementsubmit with its own modifier disabled: true for not submitting form while it's not filled:

 

CSS

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

 

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