Sass

Syntax

• Use the .scss syntax, never the original .sass syntax

  Sass has two syntaxes. The new main syntax (as of Sass 3) is known as "SCSS" (for "Sassy CSS"), and is a superset of CSS's syntax. This means that every valid CSS stylesheet is valid SCSS as well. SCSS files use the extension .scss.

  The second, older syntax is known as the indented syntax (or just "Sass"). Inspired by Haml's terseness, it's intended for people who prefer conciseness over similarity to CSS. Instead of brackets and semicolons, it uses the indentation of lines to specify blocks. Although no longer the primary syntax, the indented syntax will continue to be supported. Files in the indented syntax use the extension .sass.

  see Sass Documentation for Reference

• Order your regular CSS and @include declarations logically (see below)

Ordering of property declarations

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;
       }
   }

Variables

Prefer dash-cased variable names (e.g. $my-variable) over camelCased or snake_cased variable names.

Mixins

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 directive

@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.

Nested selectors

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.