The benefits of BEM CSS

The benefits of BEM
CSS
The benefits of BEM CSS - Bob Donderwinkel, 2014 1

Who?
• Bob Donderwinkel
• Front-end Developer at Viewbook
• Freelancer at bd creations

Who?
Our hardheaded senior front-end developer, currently
specialising in responsive HTML, CSS and mobile

Who?
Our hardheaded senior front-end developer, currently
specialising in responsive HTML, CSS and mobile
Development with Eye for Design

So what's the gist?
• What is BEM?
• What does BEM solve?
• What have I used BEM for?
• Some semi-interesting BEM tidbits

So what's the gist?
• What is BEM?
• What does BEM solve?
• What have I used BEM for?
• Some semi-interesting BEM tidbits
Mind you, I have been working with BEM CSS for about half a
year now.

So what is BEM?

Block, Element, Modifier

So what is BEM?
You might just as well call it
Module, Object, Modifier
But then you could be saying something like
I am using MOM
Which would be weird.

Unified semantic model
for markup, styles, code
and UX
— Smashing Magazine

What is BEM?
• BEM originated at Yandex
• BEM is a methodology, an abstract way of reasoning about
your interface.
• BEM captures purpose and meaning through it's CSS
classname syntax.

What is BEM?
BEM is not just for CSS. It is about communicating between
technologies and the people using them.
• Design
• Markup
• Styles
• Code

Block, Elements and Modifiers
What is that about?

Block
What is BEM?
• An independent, reusable part of your interface.
• Think: Headers, Menus, Images, etc
• Can contain Elements and other Blocks.
• Can contain one or more Modifiers
<header class='b-header'>
</header>

Element
What is BEM?
• Belongs to a Block, only has meaning there
• Can contain other Elements and Blocks.
• Think: Header sub-title, Menu item, Image caption, etc
• Can contain one or more Modifiers
<header class='b-header'>
<h1 class='b-header__title'></h1>
<p class='b-header__description'></p>
</header>

Modifiers
What is BEM?
• Belongs on a Block or Element
• Indicates a state different from the default
• Think: themes, active/inactive state, aligning, etc
• Can also be a key value pair
<header class='b-header b-header_theme_awesome'>
<h1 class='b-header__title b-header__title_large'></h1>
<p class='b-header__description'></p>
</header>

Modifiers
What is BEM?
And Modifiers are nicely groupable using CSS preprocessors
like SASS/LESS
.b-header{
&_theme_awesome{
background-color: PapayaWhip;
}
&__title {
&_large{
font-size: xx-large;
}
}
}

BEM CSS syntax
What is BEM?
The official syntax
.b-[block-name]
.b-[block-name]__[element]
.b-[block-name]__[element]_[modifier]
Pretty simple.
- Use only Latin letters, dashes and digits. Not underscores (_)
- Use underscores as the Block, Element and Modifier
separators.

BEM CSS syntax
What is BEM?
Is this okay?
.b-[block-name]__[element]__[sub-element]
.b-[block-name]_[modifier]__[element]
I have seen some conflicting articles, but probably not.

BEM CSS syntax
What is BEM?
Or roll your own syntax.
Perhaps because that better suits the CSS code base of a
project. In any case:
• Be consistent
• Keep the Block/Element/Modifier separators unique so you
can use BEM automation tools.

BEM CSS syntax
So what is BEM?
Most importantly, use meaningful BEM classnames.
If you end up with something like this, it could pretty much
defeat the point.
.b-awesome-gizmo-three__teaser_latest

What is BEM?
BEM also touches on some well known Object Oriented
Programming concepts.
Don't Repeat Yourself (DRY)
- Blocks are self contained and reusable.
Seperation of Concerns
- Using BEM CSS classes decouples your CSS from your HTML.

What is BEM?
BEM also touches on some well known Object Oriented
Programming concepts.
Single Responsibility Principle
- Blocks, Element and Modifiers focus on managing just one
thing, themselves.
Open/Closed Principle
- Modifiers often only extend on default styles and behaviour

So what does BEM CSS solve?

Chaos (Greek χάοςm
khaos) the formless or
void state preceding the
creation of .. (*)
— * = insert title of your awesome project here

Seriously, what does using BEM CSS really help with?
CSS can be wonderfully simple and easy to get started with. And
even easier to mess up on the long run.
Legacy CSS can be a royal pain.
We have all been there, or have inherited these kind of
projects to take care of.

Seriously, what does using BEM CSS really help with?
When projects scale, CSS often does not.
CSS code is easy to append, overwrite or reinvent. That can
turn in to a mess.
It is a bit like growing a new haircut every month, but only
trimming a piece of it.

It is fun until you
realise you can not
walk away from it.

BEM CSS gives you a default aproach for new projects.
• Thinking in Blocks, Elements and Modifiers helps you figure
out what your design is made of.
• BEM helps you getting started coding CSS a little bit quicker
through a standard syntax.
• BEM is about communicating between technologies and
the people using them.

For example
- Your design tells you what Blocks, Elements and Modifiers are present.
- You can use these to scaffold up a rough prototype using HTML and CSS.
- You review the prototype to see what behaviour is needed with Javascript.
- The design is tweaked and the prototype is tuned.
- Rinse and repeat.
And you're done..
..Well at least you have a better idea what your project needs
to complete ;)

Yes, yes.. Can you give me some
pratical reasons to use BEM CSS

Is there any practical stuff BEM CSS can help with?
What does BEM solve
Single CSS classname perks
Better CSS performance
• Rendering engines evaluate CSS selectors right to left.
• The less they have to evaluate, the faster it renders.

What does BEM solve
My view is that authors should not need to worry about
optimizing selectors (and from what I see, they generally don’t),
that should be the job of the engine.
~ Antti Koivisto
Luckely CSS is the least of your performance worries today.

What does BEM solve
Reduce hell in CSS specificty
• Complex CSS selectors get a higher importance. You have
to match up to this selector or use !important to override it.
• Or find the actual CSS you want to change. But the bigger
the CSS code base, the nastier that can get ;)
Using single CSS classnames makes it easier to rework your
existing CSS code base.

What does BEM solve
Global CSS reset/normalize not needed (preferably)
• Remember BEM Blocks should be independent and re-usable.
A global CSS reset/normalize does not exactly fit
that idea.
Perhaps use something like uncss in your build process to get
only the needed normalize/reset CSS into your Blocks and
Elements.

What does BEM solve
Better HTML/CSS decoupling
• Using HTML element names in your CSS selectors forces
you to update both when only one changes.
• Using just single CSS BEM classnames makes them more
portable to another HTML structure.

What does BEM solve
Project automation through BEM's predictable CSS
classnames
• Yandex has tools if you are using the standard BEM syntax
• CSS preprocessors like SASS and LESS are especially well
suited for generating BEM CSS
• And with build tools like Grunt or Gulp you can custom fit
BEM CSS automation for your own project.

But is it all utter BEM BLISS?

Utter BEM BLISS?
What about the CSS cascade?
Yes, using just single CSS classnames can feel a bit too
simplistic at times. But you can use regular CSS allong with
your BEM CSS. Or you can use SASS or LESS to @include or
@extend a CSS library.
@import "my-themes";
&_theme_awesome{
@include theme-more-awesome();
}

Utter BEM BLISS?
But I hate looking at long and ugly-ish CSS classnames
• True.. no way around that. BEM CSS can be a tad
challenging for the aesthetically inclinded code fashionista.
• But luckely the visitor of website or application will not look
in the source all too often ;)
Doesn't HTML filesize bloat with long CSS classnames?
• Yes, true. But minification and gzipping go a long way.

Utter BEM CSS BLISS?
And is BEM only useful in larger teams?
• BEM CSS certainly has extra perks in larger teams because
of the meaningful BEM CSS classnames.
• But working solo you can also use the Block, Element and
Modifier approach on a design to get started prototyping
sooner.

Utter BEM CSS BLISS?
Is BEM not yet another 'framework/methodology/
whatever' to try and get my head around?
• Yes, but it's concept and syntax is pretty straight forward to
get started with and use.
• And BEM can play pretty nicely with other CSS approaches
out there.

Utter BEM BLISS?
Other CSS approaches
Object Oriented CSS
• Separation of Structure from Skin
• Separation of Containers and Content
In essence OOCSS is about keeping CSS modular so it can be
reused and extended. This is similar to the Block, Element and
Modifier approach.

Utter BEM BLISS?
Scalable and Modular Architecture for CSS
• Guidelines how you can write and organize your CSS
SMACCS has five types of CSS categories Base, Layout,
Module, State, Theme.
The Module and State categories are simular analogue for the
Block, Element and Modifier approach.

Utter BEM BLISS?
Web components
- Polymer and Brick
Web Components are a collection of web standards focussed
on delivering self-contained and reusable custom blocks of
HTML, Javascript & CSS.
BEM CSS can be used 'inside' a Web Component just like in
any other project, and with roughly the same benefits.

Do you have some interesting
BEM CSS tidbits?

Multi modifer CSS classes
BEM tidbits
- Some Gulp.js which will rework a bit of BEM CSS so you can
use multiple Modifiers in a single BEM class
__block__element_theme-light_size-large
Wow, even longer CSS classnames...!

BEM tidbits
@bobdonderwinkel we suggest to use classic approach and write
as many classes as needed in DOM tree
@bobdonderwinkel and avoid copypaste with the help of
template engines
— Twitter #b_
Yeah, perhaps not that great an idea afterall. Fun experiment
though.

kaBEM
BEM tidbits
BEM CSS scaffolding on some steroids
kaBEM is a grunt.js environment for quick scaffolding and
tweaking of HTML with BEM CSS (Block, Element, Modifier).
- See a BEM classname in your CSS and know where to use it in your HTML
- See a BEM classname in your HTML and know where to find it in your CSS files
- See a BEM classname and know which purpose it has
But.. Disclaimer

kaBEM
BEM tidbits
Disclaimer: I created kaBEM just starting with BEM CSS. So be
mindful of some little hickups that trickeled in. For example
kaBEM uses multi element/modifier BEM CSS classnames.
.b-block__element-1__element-2
.b-block__element_modifiers-1_modifier-2
In hindsight these are a bit of a BEM CSS no-no ;) But no
worries, a new version is on the way.

BEM User Stories
BEM tidbits
User stories seem like a nice way to get going with BEM even
before you get busy with a design or prototype.
• Sum up what parts should be present (Blocks and Elements)
• Describe how these parts should look & behave (Modifiers)
• Translate these User Stories in BEM CSS classnames
• Get some coffee and start coding

Okay, round up already

Rounding up
• Thinking in Block, Elements and Modifiers helps you
interpretate a design quicker and get you started coding
sooner.
• BEM CSS classnames are about communicating, keep them
meaningful.
• BEM CSS classnames are well suited for automation
because of it's default syntax.

Good reads
Rounding up
• Scaling Down The BEM Methodology For Small Projects
• MindBEMding – getting your head ’round BEM syntax
• Organizing CSS: OOCSS, SMACSS, and BEM
• Generating BEM selectors with CSS preprocessors

Contact me!
Rounding up
• www.bdcreations.nl
• b.donderwinkel@bdcreations.nl
• Twitter
• Linkedin

The benefits of BEM CSS

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