BEM (opens new window) stands for Blocks, Elements and Modifiers. It's a methodology initially conceived by Russian tech company Yandex (opens new window), but which gained quite some traction among American & Western-European web developers as well.

As the same implies, BEM metholology is all about componentization of your HTML and CSS code into three types of components:

The goal of BEM is to keep optimize the readability, maintainability and flexibility of your CSS code. The way to achieve this, is to apply the following rules.

• Block styles are never dependent on other elements on a page
• Blocks should have a simple, short name and avoid _ or - characters
• When styling elements, use selectors of format blockname__elementname
• When styling modifiers, use selectors of format blockname--modifiername and blockname__elementname--modifiername
• Elements or blocks that have modifiers should inherit everything from the block or element it is modifying except the properties the modifier is supposed to modify

Code example

If you apply BEM to your form elements, your CSS selectors should look something like this:

.form { }                       // Block
.form--theme-xmas { }           // Block + modifier
.form--simple { }               // Block + modifier
.form__input { }                // Block > element
.form__submit { }               // Block > element
.form__submit--disabled { }     // Block > element + modifier

The corresponding HTML should look something like this:

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

Remarks

These examples are for documenting CSS-specific methodologies / design patterns.

These methodologies include but are not exclusive to the following:

• BEM (opens new window)
• OOCSS (opens new window)
• SMACSS (opens new window)

These examples are NOT for documenting CSS frameworks like Bootstrap (opens new window) or Foundation (opens new window). While you may include examples of how to apply one or more CSS methodology / design pattern with a CSS framework, those examples are to focus on the methodologies / design patterns with that particular framework and on the use of the framework itself.