The document discusses the BEM (Block, Element, Modifier) methodology for organizing CSS, HTML, and JavaScript code. It describes BEM as a system that defines three basic entities - Blocks, Elements, and Modifiers - to structure user interfaces and components. Blocks are standalone parts of the interface, Elements are integral parts of Blocks, and Modifiers define properties or states. The document provides examples and guidelines for using BEM, including naming conventions for CSS classes, benefits like improved reusability and specificity, and recommendations for file structure and build processes.

1. by Max Shirshin
Frontend Team Lead, Deltamethod
Consultant, Yandex
BEM it!
BEM Methodology for small companies
with high expectations

2. Why bother?

3. Web development needs:
● Methodologies, not frameworks
● Same entities across different
technologies
● Scalability

4. BEM to the rescue

5. What is BEM?
“BEM claims that three basic entities
(Blocks, Elements, and Modifiers)
are enough to define the way you author
HTML / CSS / JavaScript, structure code
and components, set up interaction
and scale your project to build
an industry-leading service.”

6. What is BEM?
● A methodology
“Theoretical underpinning” for methods and best
practices

7. What is BEM?
● A methodology
“Theoretical underpinning” for methods and best
practices
● Originally introduced by Yandex
— www.yandex.ru (English: www.yandex.com)
— 200+ Yandex services using full BEM stack
(methodology + tools)
— 19 million users daily audience

8. What is BEM?
● A methodology
“Theoretical underpinning” for methods and best
practices
● Originally introduced by Yandex
— www.yandex.ru (English: www.yandex.com)
— 200+ Yandex services using full BEM stack
(methodology + tools)
— 19 million users daily audience
● Used by external projects
— other services
— tools / libraries

9. Some theory :-)

10. What is BEM?
BLOCK
– Standalone part of an interface:
● button
● text field
● flyout
● heading
● menu

11. What is BEM?
BLOCK
– Standalone part of an interface:
● button
● text field
● flyout
● heading
● menu
– Re-usable in different contexts
– Self-sufficient

12. What is BEM?
ELEMENT
– An integral part of a block:
● button
● text field
● flyout
● heading
● menu

13. What is BEM?
ELEMENT
– An integral part of a block:
● button icon
● text field label
● flyout title
● heading logo
● menu item

14. What is BEM?
ELEMENT
– An integral part of a block:
● button icon
● text field label
● flyout title
● heading logo
● menu item
– No standalone meaning outside of a block
– Some blocks have no elements

15. What is BEM?
MODIFIER
– Defines property or state on a block or element:
● button
● text field
● flyout
● heading
● menu item

16. What is BEM?
MODIFIER
– Defines property or state on a block or element:
● button color
● text field disabled state
● flyout alignment
● heading level
● menu item bullet type

17. What is BEM?
MODIFIER
– Defines property or state on a block or element:
● button color
● text field disabled state
● flyout alignment
● heading level
● menu item bullet type
– Multiple modifiers may co-exist
on a single block/element

18. BEM + DOM
● Blocks are mapped to DOM

19. BEM + DOM
● Blocks are mapped to DOM
● Blocks/elems/mods are denoted
with CSS classes

20. BEM + DOM
● Blocks are mapped to DOM
● Blocks/elems/mods are denoted
with CSS classes
● DOM nodes can be “shared”: you can mix
(block1 + block2) or (element1 + block2)
on a same node

21. BEM markup forms a semantic overlay
over the existing DOM structure.
This overlay is called a BEM tree.

22. BEM HOWTO
for your beloved project
with benefits explained

23. HOWTO: HTML / CSS

24. CSS naming conventions
“BEM uses CSS class names to denote
blocks, elements and modifiers.”

25. CSS naming conventions
BLOCK
.b-button
.b-text-field
.b-flyout
.b-heading
.b-menu

26. CSS naming conventions
<ul class=”b-menu”>
<li>
<a href=”/more”>Read More</a>
</li>
<li>
<a href=”/buy”>Buy Online</a>
</li>
<li>
<a href=”/buy”>Contact</a>
</li>
</ul>

27. CSS naming conventions
ELEMENT
.b-button__icon
.b-text-field__label
.b-flyout__title
.b-heading__logo
.b-menu__item

28. CSS naming conventions
<ul class=”b-menu”>
<li class=”b-menu__item”>
<a href=”/more”>Read More</a>
</li>
<li class=”b-menu__item”>
<a href=”/buy”>Buy Online</a>
</li>
<li class=”b-menu__item”>
<a href=”/buy”>Contact</a>
</li>
</ul>

29. CSS naming conventions
MODIFIER
.b-button_color_blue
.b-text-field_disabled
.b-flyout_align_top
.b-heading_level_alpha
.b-menu__item_pos_first

30. CSS naming conventions
<ul class=”b-menu b-menu_horizontal”>
<li class=”b-menu__item”>
<a href=”/more”>Read More</a>
</li>
<li class=”b-menu__item”>
<a href=”/buy”>Buy Online</a>
</li>
<li class=”b-menu__item”>
<a href=”/buy”>Contact</a>
</li>
</ul>

31. CSS naming conventions
<ul class=”b-menu b-menu_horizontal”>
<li class=”b-menu__item b-menu__item_pos_first”>
<a href=”/more”>Read More</a>
</li>
<li class=”b-menu__item”>
<a href=”/buy”>Buy Online</a>
</li>
<li class=”b-menu__item”>
<a href=”/buy”>Contact</a>
</li>
</ul>

32. Additional notes on CSS
● Page (Document) is also a block

33. Additional notes on CSS
● Page (Document) is also a block
● Layouts are blocks (same rules apply)

34. Additional notes on CSS
● Page (Document) is also a block
● Layouts are blocks (same rules apply)
● No CSS outside of blocks

35. Additional notes on CSS
● Page (Document) is also a block
● Layouts are blocks (same rules apply)
● No CSS outside of blocks
● No common CSS resets (block independency!)

36. Additional notes on CSS
● Page (Document) is also a block
● Layouts are blocks (same rules apply)
● No CSS outside of blocks
● No common CSS resets (block independency!)
● No “shared” styles (no need)

37. Additional notes on CSS
● Page (Document) is also a block
● Layouts are blocks (same rules apply)
● No CSS outside of blocks
● No common CSS resets (block independency!)
● No “shared” styles (no need)
● DOM tree → BEM tree

38. Benefits!
Drop tag names and IDs

39. Benefits!
Drop tag names and IDs
● Faster selectors
● Re-use same semantic block on:
— <DIV class=”b-block”>
— <SPAN class=”b-block”>
— <TABLE class=”b-block”>

40. Benefits!
CSS specificity magic solved

41. Benefits!
CSS specificity magic solved
Priority of CSS rules:
by specificity first, then by rule order

42. Benefits!
CSS specificity magic solved
Priority of CSS rules:
by specificity first, then by rule order
td.odd { background-color: gray }
td.even { background-color: white }
.highlighted { background-color: yellow }
<TD class="odd highlighted">
<!-- Still gray, baby :-( -->
</TD>

43. Benefits!
CSS specificity magic solved
Priority of CSS rules:
by specificity first, then by rule order
td.odd { background-color: gray }
td.even { background-color: white }
td.highlighted { background-color: yellow }
<TD class="odd highlighted">
<!-- This works, I'm yellow now -->
</TD>

44. Benefits!
Bye-bye CSS cascade?!

45. Benefits!
Bye-bye CSS cascade?!
Only one CSS class needed to:
● style a block container
● style any element within a block
● add extras/overrides with a modifier
Doesn't it cover 90% of your styling needs?

46. Benefits!
Bye-bye CSS cascade?!
...well, not exactly.
Example of an element affected by a block modifier:
/* hide labels for disabled text fields */
.b-text-input_disabled .b-text-input__label
{
display: none;
}

47. HOWTO: JavaScript

48. JavaScript
Components → Blocks
Work with BEM tree, not DOM tree

49. JavaScript
jQuery BEM plugin
http://xslc.org/jquery-bem/
● Extends jQuery Sizzle with selectors for BEM
entities (mix them with “normal” selectors)
● Add callbacks on modifiers set/change
● Supports methods tied to blocks/elements

50. JavaScript
i-bem.js framework by Yandex + tutorial
https://github.com/toivonen/bem-js-tutorial
● First English draft docs (expect more!)
● 100% BEM-based declarative API
● Part of a larger bem-core library

51. JavaScript
Twitter Flight (used by Deltamethod)
http://flightjs.github.io
● Self-contained components, 100% event-driven
● Based on jQuery; AMD-friendly; BEM-friendly
● Built-in syntax sugar for predefined selectors
(good for BEM tree)
● [Shameless self-promo] Try Flight with Reggirt:
http://github.com/ingdir/reggirt
jQuery plugin that emulates event capturing
(opposite of event bubbling)

52. JavaScript
Twitter Flight (used by Deltamethod)
http://flightjs.github.io
● Self-contained components, 100% event-driven
● Based on jQuery; AMD-friendly; BEM-friendly
● Built-in syntax sugar for predefined selectors
(good for BEM tree)
● [Shameless self-promo] Try Flight with Reggirt:
http://github.com/ingdir/reggirt
jQuery plugin that emulates event capturing
(opposite of event bubbling)

53. Benefits!
HTML is no longer semantic.
JavaScript is.

54. HOWTO: Design / UX

55. BEM is the universal language
for developers and designers,
the bridge across technology gaps.

56. UX + Frontend
● Design a style guide

57. UX + Frontend
● Design a style guide
● Keep it up-to-date

58. UX + Frontend
● Design a style guide
● Keep it up-to-date, always!

59. UX + Frontend
● Design a style guide
● Keep it up-to-date, always!
● Build new screens quickly

60. UX + Frontend
Build your block library.
The code itself is the styleguide.

61. Benefits!
● Prototyping mapped to code from day 1

62. Benefits!
● Prototyping mapped to code from day 1
● Code mapped to prototypes from day 1

63. HOWTO: File structure

64. File and folder structure
Flat block structure with a folder for each block.
Simple structure for BEM beginners:
/b-block
block.css
block.js
block.tpl
...whatever you need

65. File and folder structure
Advanced structure to expose semantics
/block
/css
block.css
block__elem1.css block__elem2.css
block_mod.css
/js
block.js
/template
block.tpl block__elem1.tpl block__elem2.tpl

66. File and folder structure
Advanced structure to expose semantics
/block
/css
block.css
block__elem1.css block__elem2.css
block_mod.css
/js
block.js
/template
block.tpl block__elem1.tpl block__elem2.tpl

67. Benefits!
● Unified structure for automation
● Redefinition levels: different libraries, same
structure
/common
/b-block
/css
block.css
/template
block.tpl
/app1
/b-block
/css
block__elem1.css
/js
block.js
/template
block__elem1.tpl
+

68. Build process and deployment
Use a build tool!
Borschik:
an open-source build tool by Yandex
Code:
https://github.com/bem/borschik
English docs:
http://bem.info/articles/borschik

69. max.shirshin@gmail.com http://gplus.to/ingdir
@ingdir maxshirshin
Thank you!