To BEM or Not to BEM

Written by Łukasz Kliś

I’ve been working as a front-end developer since 2007. Since then, a lot of frameworks, grid systems and methodologies have been developed to help developers’ worklife. Bootstrap, Foundation, Semantic UI—those are examples of only some of those. Tools like these have allowed us to rapidly create prototypes or web applications. Doing front-end became easy. Did it really?

Hello Bootstrap

Let’s take a closer look at one of the most famous CSS framework—Bootstrap. You’ve picked a task where you’re supposed to create a button on your site.

<button class="btn">
  Click me

Save a HTML file, refresh the browser and voilá! A nice button shows up.

A basic button from Bootstrap

Moving on to the next task, you’ve been asked to make the button larger, make it more prominent and fill the entire width of the container. This requires more work—you may think. Quick look at Bootstrap’s documentation and all you have to do is add some extra classes:

<button class="btn btn-lg btn-primary btn-block">
  Click me

Once again, save, refresh and task seems to be done.

A more prominent button

Quick Fixes

You keep working on your project for the next couple of months. Bootstrap, as a CSS framework of choice, seems to be perfect. Everybody is happy—a client, a PO and you.

Suddenly, you’ve been assigned to a task where you have to fix one thing in your layout. Eventually, you’ll end up with code like below:

<button class="btn btn-lg btn-primary btn-block btn-cta btn-orange">
  Click me

After pushing such code to the repository, you start to wonder what’s the purpose of your work and life. Is it really something that you want to do? Aren’t out there any better solutions which may help you keep the codebase clean and easier to maintain?

BEM to the Rescue!

BEMBlock Element Modifier—a methodology created by Yandex in 2005 (yeah, BEM is already 10 years old!). The methodology comes with some guidelines on how to name elements in your templates and stylesheets.

What’s a block, an element and a modifier? Let’s take a look at the example layout below:

An example of a page layout Source:

B for Block

While implementing beautiful designs provided to you by the designer, you try to split the page into modules. You do your best to find common patterns between them, so you could re-use as much code as possible. Such a module is what we call a block. You can find a few of them in the example layout:

An example layout with blocks marked Source:

As you can see, blocks can be nested, as shown in the screenshot below:

A closer look at the header block Source:

A header block contains blocks for logo, search form, navigation menu and user’s login.

E for Element

What about the elements? Elements are always a part of a block and you should never use them outside of it. Where can you find elements in the example layout? Let’s take a closer look at the navigation in the header:

A closer look at the header’s navigation block Source:

M for Modifier

As mentioned before, you always do your best to find common parts of different blocks across the page, however sometimes you have to apply some custom styles for a specific block. Let’s take a look at the navigation menus in the header and the footer:

A closer look at the header’s and footer’s navigation block Source:

Both of the navigation menus have a common part, i.e. float: left; and perhaps a color of a link. As you can see, the navigation menu in the bottom of the page has some additional spacing applied between the links and that’s when a modifier comes in handy.


BEM doesn’t force you to use a specific syntax. However, the creator of the methodology (Yandex) proposed a syntax that you could follow.

The Yandex Way

<div class="block__element"></div>
<div class="block__element_mod_value"></div>

Yandex uses double underscore character to distinguish a block from an element, and _mod_ to distinguish a modifier from a block or an element.

<div class="block__element"></div>
<div class="block__element

Harry Roberts’ uses double underscore character to distinguish a block from an element, and double dash character to distinguish a modifier from a block or an element. Also, for the sake of modifiers’ readability, a block or an element to which a modifier is applied, is duplicated.

Your Way

As mentioned before, BEM offers some freedom in terms of separators choice. Whatever suits you best and is valid with W3C’s specification is fine. For instance, when you’re a fan of emoticons, why not go for the syntax below?

<div class="block-_-element"></div>
<div class="block-_-element-o_O-modifier"></div>

BEM in Practice

Take a look at the follow up posts with case study of refactoring front-end code at Wimdu:

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