A mostly reasonable approach to CSS and Sass

1. Terminology
   • Rule Declaration
   • Selectors
   • Properties
2. CSS
   • Formatting
   • Comments
   • OOCSS and BEM
   • ID Selectors
   • JavaScript hooks
   • Border
3. Sass
   • Syntax
   • Ordering
   • Variables
   • Mixins
   • Extend directive
   • Nested selectors
4. Translation

A “rule declaration” is the name given to a selector (or a group of selectors) with an accompanying group of properties. Here's an example:

.listing {
  font-size: 18px;
  line-height: 1.2;
}

In a rule declaration, “selectors” are the bits that determine which elements in the DOM tree will be styled by the defined properties. Selectors can match HTML elements, as well as an element's class, ID, or any of its attributes. Here are some examples of selectors:

.my-element-class {
  /* ... */
}

[aria-hidden] {
  /* ... */
}

Finally, properties are what give the selected elements of a rule declaration their style. Properties are key-value pairs, and a rule declaration can contain one or more property declarations. Property declarations look like this:

/* some selector */ {
  background: #f1f1f1;
  color: #333;
}

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• Use soft tabs (2 spaces) for indentation
• Prefer dashes over camelCasing in class names.
  • Underscores and PascalCasing are okay if you are using BEM (see OOCSS and BEM below).
• Do not use ID selectors
• When using multiple selectors in a rule declaration, give each selector its own line.
• Put a space before the opening brace { in rule declarations
• In properties, put a space after, but not before, the : character.
• Put closing braces } of rule declarations on a new line
• Put blank lines between rule declarations

Bad

.avatar{
    border-radius:50%;
    border:2px solid white; }
.no, .nope, .not_good {
    // ...
}
#lol-no {
  // ...
}

Good

.avatar {
  border-radius: 50%;
  border: 2px solid white;
}

.one,
.selector,
.per-line {
  // ...
}

• Prefer line comments (// in Sass-land) to block comments.
• Prefer comments on their own line. Avoid end-of-line comments.
• Write detailed comments for code that isn't self-documenting:
  • Uses of z-index
  • Compatibility or browser-specific hacks

We encourage some combination of OOCSS and BEM for these reasons:

• It helps create clear, strict relationships between CSS and HTML
• It helps us create reusable, composable components
• It allows for less nesting and lower specificity
• It helps in building scalable stylesheets

OOCSS, or “Object Oriented CSS”, is an approach for writing CSS that encourages you to think about your stylesheets as a collection of “objects”: reusable, repeatable snippets that can be used independently throughout a website.

• Nicole Sullivan's OOCSS wiki
• Smashing Magazine's Introduction to OOCSS

BEM, or “Block-Element-Modifier”, is a naming convention for classes in HTML and CSS. It was originally developed by Yandex with large codebases and scalability in mind, and can serve as a solid set of guidelines for implementing OOCSS.

• CSS Trick's BEM 101
• Harry Roberts' introduction to BEM

We recommend a variant of BEM with PascalCased “blocks”, which works particularly well when combined with components (e.g. React). Underscores and dashes are still used for modifiers and children.

Example

// ListingCard.jsx
function ListingCard() {
  return (
    <article class="ListingCard ListingCard--featured">

      <h1 class="ListingCard__title">Adorable 2BR in the sunny Mission</h1>

      <div class="ListingCard__content">
        <p>Vestibulum id ligula porta felis euismod semper.</p>
      </div>

    </article>
  );
}

/* ListingCard.css */
.ListingCard { }
.ListingCard--featured { }
.ListingCard__title { }
.ListingCard__content { }

• .ListingCard is the “block” and represents the higher-level component
• .ListingCard__title is an “element” and represents a descendant of .ListingCard that helps compose the block as a whole.
• .ListingCard--featured is a “modifier” and represents a different state or variation on the .ListingCard block.

While it is possible to select elements by ID in CSS, it should generally be considered an anti-pattern. ID selectors introduce an unnecessarily high level of specificity to your rule declarations, and they are not reusable.

For more on this subject, read CSS Wizardry's article on dealing with specificity.

Avoid binding to the same class in both your CSS and JavaScript. Conflating the two often leads to, at a minimum, time wasted during refactoring when a developer must cross-reference each class they are changing, and at its worst, developers being afraid to make changes for fear of breaking functionality.

We recommend creating JavaScript-specific classes to bind to, prefixed with .js-:

<button class="btn btn-primary js-request-to-book">Request to Book</button>

Use 0 instead of none to specify that a style has no border.

Bad

.foo {
  border: none;
}

Good

.foo {
  border: 0;
}

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• Use the .scss syntax, never the original .sass syntax
• Order your regular CSS and @include declarations logically (see below)

1. Property declarations

   List all standard property declarations, anything that isn't an @include or a nested selector.

   .btn-green {
     background: green;
     font-weight: bold;
     // ...
   }

2. @include declarations

   Grouping @includes at the end makes it easier to read the entire selector.

   .btn-green {
     background: green;
     font-weight: bold;
     @include transition(background 0.5s ease);
     // ...
   }

3. Nested selectors

   Nested selectors, if necessary, go last, and nothing goes after them. Add whitespace between your rule declarations and nested selectors, as well as between adjacent nested selectors. Apply the same guidelines as above to your nested selectors.

   .btn {
     background: green;
     font-weight: bold;
     @include transition(background 0.5s ease);
   
     .icon {
       margin-right: 10px;
     }
   }

Prefer dash-cased variable names (e.g. $my-variable) over camelCased or snake_cased variable names. It is acceptable to prefix variable names that are intended to be used only within the same file with an underscore (e.g. $_my-variable).

Mixins should be used to DRY up your code, add clarity, or abstract complexity--in much the same way as well-named functions. Mixins that accept no arguments can be useful for this, but note that if you are not compressing your payload (e.g. gzip), this may contribute to unnecessary code duplication in the resulting styles.

@extend should be avoided because it has unintuitive and potentially dangerous behavior, especially when used with nested selectors. Even extending top-level placeholder selectors can cause problems if the order of selectors ends up changing later (e.g. if they are in other files and the order the files are loaded shifts). Gzipping should handle most of the savings you would have gained by using @extend, and you can DRY up your stylesheets nicely with mixins.

Do not nest selectors more than three levels deep!

.page-container {
  .content {
    .profile {
      // STOP!
    }
  }
}

When selectors become this long, you're likely writing CSS that is:

• Strongly coupled to the HTML (fragile) —OR—
• Overly specific (powerful) —OR—
• Not reusable

Again: never nest ID selectors!

If you must use an ID selector in the first place (and you should really try not to), they should never be nested. If you find yourself doing this, you need to revisit your markup, or figure out why such strong specificity is needed. If you are writing well formed HTML and CSS, you should never need to do this.

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This style guide is also available in other languages:

• Bahasa Indonesia: mazipan/css-style-guide
• Chinese (Traditional): ArvinH/css-style-guide
• Chinese (Simplified): Zhangjd/css-style-guide
• French: mat-u/css-style-guide
• Japanese: nao215/css-style-guide
• Korean: CodeMakeBros/css-style-guide
• Portuguese: felipevolpatto/css-style-guide
• Russian: Nekorsis/css-style-guide
• Spanish: ismamz/guia-de-estilo-css
• Vietnamese: trungk18/css-style-guide
• Italian: antoniofull/linee-guida-css

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There are many style guidelines out there. Airbnb's CSS styleguideline seems to hit most of the important marks. Its missing some important things

Resources used:

• https://vanseodesign.com/css/sass-directory-structures/
• https://smacss.com/
• https://github.com/stubbornella/oocss/wiki
• http://getbem.com/introduction/

Folder structures depend on each project. For small - medium projects, it makes much more sense to have individual files over folders. The following layout is generally widely accepted:

• _base.scss - HTML element modification and vendor changes (overwrites to Bootstrap)
• _layout.scss - Macro layout
• _modules.scss - micro layout

Depending on project, a few other style guidelines standout

• _page.scss - Specific page layouts
• _shame.scss - Poorly made SCSS
• _state.scss - State changes according to OOCSS principles
• _vendor.scss - changes from framework

Its also important to note that having these folder names are named so that if you read the files from top to bottom, they are sorted alphabetically by priority / specificity

Some similarities occur in all 3 frameworks and can be applied. The following conventions is what I think should be used in this exact order

• SMASCSS folder structures (highlighted above)
• OOCSS principles
• BEM for everything else

OOCSS suggests the following principles, which BEM adheres too

• Seperation of Structure from Skin (OOCSS)
• Seperation of content from containers

BEM suggests using the following principles

• Block
• Element
• Modifier

The major difference between OOCSS and BEM is the naming implementation. They both suggest the same thing.

• Block-Element is seperation of contain from containers
• Modifier is seperation of structure

OOCSS leaves the implementation up to you but BEM is opinionated

The following things set of rules should be applied when deciding on a CSS architecture. For instance, take an ecommerce project as an example running a bootstrap setup

• What are all the unique pages used? (checkout, category landing page, product-grid page, product-landing page etc). These should be used a PUG inheritance style layout
• What are the reusable elements across pages? - these go under _layout.scss.
• What about micro elements across pages? - Micro elements may not be be needed if they can be adopted from a framework like bootstrap. If this is the case, changes should go under _vendor.scss.

Its important to note that if using a framework, you should compartmentalize where and when it is used. This way, in the event of a complete overhaul of your CSS / HTML, refactoring is easy.

A few conventions should be applied when doing this

Don't

• Rely on too many framework classes. Use as few as possible. E.g. Bootstrap's row, columns, and rows are sufficient for most things.

Do

• Override CSS of bootstrap framework. E.g. bg-background you can override it with background-color: yellow !important under _vendor.scss. This makes customization much easier than modifying the original SCSS files. Later, you can refactor and identify what elements you used quickly by looking at _vendor.scss
• Document which classes you use in _vendor.scss

Next, there should be some sort of convention for determining how styles are created. These questions should be the following:

There's always a tradeoff between how easy the HTML is to read VS how easy the CSS is to understand.

BEM main disadvantage is creating excessively long classes and state modifiers

Ideally, it is best to get both semantic CSS and semantic HTML

• Semantic CSS → Use global state modifiers whenever possible following OOCSS
• Semantic HTML → Use bootstraps opinionated framework for now