lean-bem

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name: lean-bem description: Naming convention and CSS architecture rules for writing stylesheets and HTML class names with Lean BEM. Use when writing, reviewing, or refactoring CSS/HTML: naming classes, creating components, or organizing stylesheets in a project that follows Lean BEM (or BEM).

Lean BEM

Lean BEM is a readability-driven, alternate naming convention to classic BEM. The interface is sliced into independent blocks, which are formed by elements; modifiers adjust the appearance, state, or behavior of either. Apply these rules whenever you write or review HTML class names and CSS.

Naming scheme

Entity	Pattern	Example
Block	block	.button, .icon
Element	block_element	.button_label, .icon_download
Modifier	-modifier	.-big, .-disabled

Each separator has exactly one meaning:

• A single hyphen (-) joins composed words: block date-picker, element date-picker_nav-button, modifier -switch-theme.
• A single underscore (_) separates the block name from the element name; it has no other use.
• A leading hyphen (-) marks a modifier: an extra class added next to the block or element class it modifies, never used by itself.

<!-- Classic BEM -->
<button class="button button__primary button__primary_disabled button_big">…</button>

<!-- Lean BEM -->
<button class="button button_primary -disabled -big">…</button>

In the Lean BEM line, button is the block, button_primary is an element (a variant of the button), and -disabled and -big are modifiers. Watch the underscore when translating from classic BEM: there, a single underscore marks a modifier (button_big); in Lean BEM it marks an element, and modifiers are the leading-hyphen classes.

Block

An independent, reusable component.

• Name it by purpose, answering “What is it?” (button, icon), never by state or appearance (red, big 🚫).
• Prefer short names: block over long-block over super-long-block.
• Blocks can be nested inside other blocks (see Blocks inside blocks).
• A block must not influence its environment: never set external geometry or positioning on the main block selector. The parent positions it instead.

<!-- `button` block -->
<button class="button">…</button>

.button {…}

Element

A composite part of a block that can't be used separately from it.

• Name it by purpose, answering “What is this?” (item, text, label), not by state or appearance.
• The full class name is block_element, with a single underscore: button_label is the label element of the button block.
• The class name always starts with the block name, no matter how deep the element's tag sits in the DOM. Elements belong to the block, never to another element, so names never chain: even if element-two's tag is nested inside element-one's, its class is block_element-two, not block_element-one_element-two 🚫.
• Style every element with a flat, single-class selector. Elements nested in the DOM must not be nested in the stylesheet: .button_label {…} ✅, .button .button_label {…} 🚫. Flat selectors keep specificity low and overrides simple.
• An element can sit on the same tag as its block when it names a variant of it (class="button button_secondary"); see Element or modifier?.

<!-- `button` block with its `secondary` variant element -->
<button class="button button_secondary">
  <!-- `label` element of the `button` block -->
  <span class="button_label">Download</span>
  <!-- nested `icon` block with its `download` element -->
  <span class="icon icon_download"></span>
</button>

.button {…}
.button_secondary {…}
.button_label {…}
.icon_download {…}

Modifier

A class that defines the appearance, state, or behavior of a block or element.

• It describes appearance, such as -big or -dark (“What size? Which theme?”); state, such as -disabled or -focused (“How is it different from the others?”); or behavior, such as -switch-theme (“How does it respond to the user?”).
• It never stands alone: a modifier changes an entity, it doesn't replace it. class="button -big" ✅, class="-big" 🚫.
• In CSS, chain the modifier to the class it modifies, with no space between them:
  • .button.-big {…} ✅ (block with modifier).
  • .button_primary.-disabled {…} ✅ (element with modifier).
  • .button .-big {…} 🚫: the space turns it into a descendant selector, which styles any inner entity carrying -big (like the icon below) instead of the button itself.
• Modifier names are generic on purpose, so they can be reused across blocks with different styles (.button.-big, .icon.-big). That's also why each rule must be scoped to a parent class: a bare .-big {…} would leak into every block that uses the name.

<!-- `button` block with the `-big` modifier -->
<button class="button button_secondary -big">
  <span class="button_label">Download</span>
  <!-- the nested `icon` block carries its own `-big` -->
  <span class="icon icon_download -big"></span>
</button>

.button.-big {…}
.icon.-big {…}

Element or modifier?

Both can qualify a block, so choose by what the class expresses:

• Element: a named variant that defines what the entity is (“What is this?”), like button_primary and button_secondary.
• Modifier: a composable adjustment that can be added or removed without changing what the entity is, like -big, -disabled, and -dark.

A big, disabled primary button is class="button button_primary -big -disabled".

CSS rules

• Selectors target classes only, never tags, IDs, or attributes: .button {…} ✅; button {…}, #button {…}, [type="submit"] {…} 🚫.
• Default to flat, single-class selectors. Reserve descendant selectors for a parent styling a nested block (typically to position it), and stop at two levels: .page_container .header {…} ✅, .button .icon_download {…} ✅, anything deeper 🚫.
• Prefer composition over inheritance: build variations by combining block, element, and modifier classes in the markup, not by extending or duplicating rules in the stylesheet. Composition keeps code uncoupled and flexible.
• Follow a consistent code formatting standard (@mdo's Code Guide is the suggested one).

Blocks inside blocks

Nesting blocks is fine and expected. Blocks are functionally independent, so they can be moved around freely to compose UI patterns, which only works if a block never positions itself. The parent sets the positioning of the blocks nested inside it.

<!-- `page` block -->
<body class="page">
  <!-- `container` element of the `page` block -->
  <div class="page_container">
    <!-- nested `header` block -->
    <header class="header">…</header>
    <!-- nested `footer` block -->
    <footer class="footer">…</footer>
  </div>
</body>

/* The parent's element positions each nested block ✅ */
.page_container .header { float: left; }
.page_container .footer { float: right; }

/* Never set positioning on the main block selector 🚫 */
.header { float: left; }

If .header floated itself, it would carry that positioning into every context where it's reused. Owned by .page_container, the positioning applies only there.

Quick checklist

When writing or reviewing code, verify:

1. Class names describe purpose (button, button_label), not state or appearance (red-button 🚫).
2. Element classes have exactly one underscore, block name first (block_element); elements never chain (block_element-one_element-two 🚫).
3. Modifiers start with -, never appear alone in HTML, and are chained without a space in CSS (.block.-modifier, never .block .-modifier).
4. No positioning on a main block selector; the parent positions nested blocks.
5. Selectors target classes only; elements are styled flat; descendant selectors go at most two levels, reserved for a parent styling a nested block.

Further reading

• Lean BEM repository
• BEM Methodology
• BEM For Beginners: Why You Need BEM