A mostly reasonable approach to CSS and Sass

1. Terminology - Rule Declaration - Selectors - Properties
2. CSS - Formatting - Comments - ID Selectors - JavaScript hooks - Border
3. Sass - Ordering - Variables - Mixins - Nested selectors - File Structure - Responsive SCSS

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;
}

• Prefer dashes over camelCasing in class names.
• 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

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.

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-contact-form">Contact Us</button>

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

Bad

.foo {
  border: none;
}

Good

.foo {
  border: 0;
}

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.

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.

###File Structure

We prefer to break up our .sass across multiple specifically named files

|--scss/
   |--modules/
   |--pages/
   |--partials/
   |--main.scss

• modules - a folder that contains functions and mixins and variables. Things that are included into other SCSS files.
• pages - a folder that contains one .scss file for each page - generally only specific css to that page. (i.e. home.scss, about.scss, gallery.scss, etc..) Essentially the structure of the site.
• partials - a folder that contains things/elements that are considered part of the site's skin - i.e. buttons, inputs, media. These are reusable throughout the site.

###Responsive SCSS

We generally prefer to have each element/rule to have it's own responsive rule rather than blocks for each size. This might seem counter-intuitive, but makes for easier editing.

Bad

.avatar{
    border-radius:50%;
    border:2px solid white; 
}
.images {
    border: 5px;
    width:50%;
    display: inline-block;
}
#page {
  padding: 10px;
}

@media #{$xlarge-up} { 
  .avatar{
    border-radius:0;
    border:0;
  }
  .images {
      width:100%;
      display: block;
  }
  #page {
    padding: 20px;
  }
}

Good

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

    @media #{$xlarge-up} { 
      border-radius:0;
      border:0;
    }
}
.images {
    border: 5px;
    width:50%;
    display: inline-block;

    @media #{$xlarge-up} { 
      width:100%;
      display: block;
    }
}
#page {
  padding: 10px;

  @media #{$xlarge-up} { 
    padding: 20px;
  }    
}