CSS Bliss

A CSS style guide for small to enormous projects, without all that pomp and cruft. Many ideas borrowed from BEM, SMACSS, OOCSS, SUITECSS. This style guide uses SCSS. However, the only truly beneficial preprocessor-specific features we discuss are variables, mixins, and partials. Therefore, this guide will be useful for any preprocessor or no preprocessor at all as there is very little focus on features that aren't already part of vanilla CSS.

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There is now a Walkthrough. If you have never used BEM, SMACSS, or similar, reading the Walkthrough is highly recommended.

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• General
• Naming
• Building Blocks
• Encapsulation
• Semantics
• Comments
• %placeholder
• @extend
• @mixin
• $variable
• DRY
• Directory Structure and File Naming
• Positioning a Module inside of a Module
• z-index
• Namespacing
• Linter
• Solving Complexity

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If you have questions, comments, or suggestions please open an Issue. And of course, PRs are welcome.

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General

• Use class selectors instead of element or attr selectors in most cases.
• SASS gives us too much power. In part the purpose of this guide is to restrict our use of that power.
• Try to avoid nesting, except as described in the DRY section.
• Keep Modules small. When in doubt, create a new Module instead of bloating an existing Module.
• A class name will almost never have more than 3 dashes, ie: .MyModule-myElement--myModifier
• Be sure to click the [ pen ] links as you encounter them below.

Naming

TitleCase Modules, camelCase Elements. why-not-dashes ? Because cased names are more readable (very objective explanation, I know). Furthermore, if we see a class name that doesntStartWithTitleCase we know that it's not a module.

Note: The following example does not match the conventions laid out in the DRY section because this is the compiled CSS code, not the SASS code.

.MyModule {
  ...
}

.MyModule-myElement {
  ...
}

.MyModule-myElement--modifier {
  ...
}

.MyModule--modifier .MyModule-myElement {
  ...
}

.MyModule.isState {
  ...
}

.mySimpleRule {
    ...
}

Building Blocks

Module

• .TitleCase
• Self-contained
• Cannot be @extended to create Module Modifiers
• Most Modules should not set their own width, margin, and positioning. By authoring a module to be full-width or inline, it can better adapt to the dimensions of an ancestral context. (source)
  • No margin
  • No top, left, right, bottom
  • 100% width, or auto width

Module Modifier

[ pen ]

• --camelCase
• Could possibly define margin, top, left, right, or bottom but should probably be avoided in most cases.
• Subclasses a Module
• Do not @extend a Module to create a Module Modifier
• Do not @extend a Module Modifier to create another Module Modifier

Element

• -camelCase
• Each element has an associated module ie: .MyModule-myElement
• Nesting Modules should not effect the appearance of any Element.
• Apply at most one Element class per DOM element.

Element Modifier

• --camelCase
• Each modifier has an associated Element ie: .MyModule-myElement--myModifier
• Nesting Modules should not effect the appearance of any Element Modifier.
• Subclasses an Element

State

• .isCamelCase (formerly .is-camelCase)
• Often, but not necessarily used in conjunction with JavaScript
• No style except in context with another rule. For example: .MyModule.isState, .MyModule-myElement.isState, .MyModule-myElement--myModifier.isState, .isState .MyModule-myElement, etc.

Utility Classes (aka Simple Rules)

• .camelCase
• Utility Classes .may-containDashes when the dashed word might imply very similar meaning as a function argument does in javascript. A good use-case for dashed utility classes are device-specific classes such as .col2-mobile, .col2-tablet, etc.
• These rules should be completely flat. They include what are often called utility classes and layout rules.

Encapsulation

Modules promote strict encapsulation. We achieve encapsulation thusly:

• No module may ever effect any other module by creating a selector that reaches inside of another module.
• All classes inside of a module file are namespaced by the module's name (with the notable exception of state (.is) classes)
• To achieve the strictest form of encapsulation, no DOM element should ever be assigned module-namespaced classes from more than one module.

Semantics

When choosing class names, try to ignore function, and concentrate on style. Just because we have a section on our website named music doesn't mean that we should name our music-related card Module MusicCard. Sure, we can name it MusicCard if similar cards do not appear anywhere else on the website. Otherwise, name it Card instead. (Since we probably can't know for sure whether the same card might later be added to a new section of the site, when naming classes make a judgement call based on the information available at the time and don't try to plan too far into the future.)

And now that we have our Card module, if we need a modifier for a green-tinted card in the section we're currently working on (which happens to be the music section) name it Card--greenTint. If your modifier truly is specific to the music section then Card--music is OK when a more stylistically descriptive name doesn't present itself.

There is a lot of seemingly conflicting information about CSS semantics. Some people say to name your class by function like this:

<div class="FacebookBtn">FB</div>

instead of

<div class="Btn">FB</div>

However, we favor the second approach. Using the .FacebookBtn class name is convenient when you're using a library like jQuery because it keeps your markup and jQuery selector code semantically correct. However, with HTML5 and modern frameworks like angularjs the markup is already semantically correct via the use of custom element naming, attributes, and event handlers which declaratively describe the content and its function.

What about our Jasmine unit tests which are heavy with jQuery selectors? If class names are un-semantic does that force us to write unit tests that break when simple stylistic changes are made to the interface? Not if we favor the use of attribute and element selectors in our unit tests which generally means the tests will only break when functionality changes.

Comments

Above each Module, describe the purpose of the Module, as well as it's scope. Be restrictive and specific so that when someone else looks at it and needs to add some styling, they will know if they should add-on to the Module or create a new one.

// A floating dialog with dropshadow, rounded borders, and a header.
// Includes the header text, but not any buttons and none of the other content inside of the dialog.
.PopupDialog {
    ...
}

Note that in the example above, the comment answers the question, what is and isn't a PopupDialog Module allowed to style?. Later, if one of our teammates wants to add a close button to this dialog, they can read this comment and determine that the close button should probably be a Module in it's own right, and not an element of the PopupDialog module.

%placeholder

• Naming: %MyModule-placeholderDescriptor
• Restrictions:
  • Placeholders may only be declared inside of Modules. (why?)
  • Modules may only use placeholders declared inside of them.
  • A placeholder declared inside of a Module may not be used anywhere else.

The next section discusses why we should avoid using @extend. However if we do use @extend despite our trepedations, we will try to restrict its use to placeholders. And as we should with any @extended class, keep %placeholders flat, here's why.

%placeholder {
  // avoid nesting .anyClassRules at any cost...
  .dontDoThis { ... }
}

%placeholders should be small pieces of reusable styling, and they should do one thing and do it well.

@extend

Don't @extend Modules to create Module Modifiers because

1. keeping modules totally flat may be near impossible
2. @extend can result in more code than simple subclassing
3. @extend is incompatible with media queries
4. @extend makes understanding the cascade of SCSS code very difficult

What about using @extend in other rules? In most cases, using @extend can lead to confusion and should probably be avoided.

@mixin

When a @mixin is declared inside of a Module:

• Naming: @MyModule-mixinDescriptor
• It may not be used outside of the Module in which it is declared.

When a @mixin is declared outside of a Module:

• Naming: @mixinDescriptor
• It should not generate complex nested structures.

$variable

When a $variable is declared inside of a Module:

• Naming: $MyModule-variableDescriptor
• It may not be used outside of the Module in which it is declared.

The purpose of this rule is to make it easy to identify local vs global variables.

DRY

[ pen ]

.MyModule {
  ...
  &-myElement {
    ...
  }
  
  &-myOtherElement {
    ...
  }
  
  &-myOtherElement--myModifier {
    ...
  }
  
  &-myOtherElement--anotherModifier {
    ...
  }
}

and module modifiers:

.MyModule--myModifier {
  ...
  .MyModule-myOtherElement {
    ...
  }
    
  .MyModule-myOtherElement--anotherModifier {
    ...
  }
}

A downside is that doing a full-text search for a class won't take us where we need to go, but if the naming convention is well-established we'll have that in mind when searching anyway.

This DRY approach prevents @extending Elements and Element Modifiers. This is good because @extending these nested classes creates confusing and difficult to maintain code.

Directory Structure and File Naming

Create a new file for each Module and it's Module Modifiers and save inside of the modules directory. Use TitleCase naming for module files and dashed-names for everything else. The following example suggests putting non-module rules alongside application.scss in the css dir.

css
 ├─── modules
 |     ├──── _PopupDialog.scss
 |     ├──── _Btn.scss
 |     └──── _ElmInfo.scss
 ├─── _base.scss
 ├─── _colors.scss
 ├─── _mixins.scss
 ├─── _zindex.scss
 └─── application.scss

In the example above, the sass compiler is compiling application.scss and all of the other files are being @imported from application.scss (is this the best way?).

Use the SASS underscore naming convention whereby all partials begin with underscore.

Positioning a Module inside of a Module

We will inevitably want to nest Modules inside of modules. There are various ways that we could possibly position one Module inside of another. In most cases we should subclass the child Module with an Element class in the parent Module.

Preferred approach using Element

[ pen ]

Here we wrap the .Btn element with an .PopupDialog-closeBtn element:

SCSS

// modules/_Btn.scss
.Btn {
  ...
}

// modules/_PopupDialog.scss
.PopupDialog {
  ...
  &-closeBtn {
    position: absolute;
    top: 10px;
    right: 10px;
  }
}

Markup

<div class="PopupDialog">
  ...
  <div class="PopupDialog-closeBtn">
    <button class="Btn"><i class="closeIco"></i> close</btn>
  </div>
</div>

Note that the above approach is extremely flexible. If we want to swap out the Btn module for a different button module, it won't require any CSS changes. (And if we have a pattern library, such a change will be as simple as copy-and-pasting some markup.)

Note also that if we wish to completely avoid Module file load-order specificity bugs and/or we need to load multiple CSS files asynchronously, PopupDialog-closeBtn shouldn't subclass .Btn, but instead wrap it inside of another <div> as we've done here.

Alternate approach using Module Modifier

[ pen ]

Here we subclass .Btn with .Btn--pullRight:

SCSS

.Btn {
  ...
}

.Btn--pullRight {
    position: absolute;
    top: 10px;
    right: 10px;
}

Markup

<div class="PopupDialog">
  ...
  <button class="Btn Btn--pullRight"><i class="closeIco"></i> close</btn>
</div>

Note that the above approach is inflexible because in the future we won't be able to easily swap out the button without refactoring the css. Also, since it creates an unpredictable parent-child module positioning relationship the code is fragile which will make it more difficult to maintain.

z-index

All z-index rules reference $zindexVariables in _zindex.scss. This creates a central place to manage z-indexes accross the application.

Namespacing

I don't like how it negatively effects readability, but if we need to namespace, prepend a lowercase two or three letter abbreviation.

.ns-MyModule {
  ...
}

Linter

This tool does not exist, but it would be cool if it did.

• Confirm proper naming
• Enforce flat @extends

Solving Complexity

solving-complexity.md is another document in this repo inspired by the challenges identified by Facebook's vjeux. solving-complexity presents some additional guidelines, which, when applied in addition to this guide help to solve problems faced by the most complex websites.

Additional Resources

• include-media - A nice way to handle @media queries.