Dropbox (S)CSS Style Guide

“Every line of code should appear to be written by a single person, no matter the number of contributors.” —@mdo

General

Don’ts

• Avoid using HTML tags in CSS selectors
  • E.g. Prefer .o-modal {} over div.o-modal {}
  • Always prefer using a class over HTML tags (with some exceptions like CSS resets)
• Don't use ids in selectors
  • #header is overly specific compared to, for example .header and is much harder to override
  • Read more about the headaches associated with IDs in CSS here.
• Don’t nest more than 3 levels deep
  • Nesting selectors increases specificity, meaning that overriding any CSS set therein needs to be targeted with an even more specific selector. This quickly becomes a significant maintenance issue.
• Avoid using nesting for anything other than pseudo selectors and state selectors.
  • E.g. nesting :hover, :focus, ::before, etc. is OK, but nesting selectors inside selectors should be avoided.
• Don't !important
  • Ever.
  • If you must, leave a comment, and prioritise resolving specificity issues before resorting to !important.
  • !important greatly increases the power of a CSS declaration, making it extremely tough to override in the future. It’s only possible to override with another !important declaration later in the cascade.
• Don’t use margin-top.
  • Vertical margins collapse. Always prefer padding-top ormargin-bottom on preceding elements
• Avoid shorthand properties (unless you really need them)
  • It can be tempting to use, for instance, background: #fff instead of background-color: #fff, but doing so overrides other values encapsulated by the shorthand property. (In this case, background-image and its associative properties are set to “none.”
  • This applies to all properties with a shorthand: border, margin, padding, font, etc.

Spacing

• Four spaces for indenting code
• Put spaces after : in property declarations
  • E.g. color: red; instead of color:red;
• Put spaces before { in rule declarations
  • E.g. .o-modal { instead of .o-modal{
• Write your CSS one line per property
• Add a line break after } closing rule declarations
• When grouping selectors, keep individual selectors on a single line
• Place closing braces } on a new line
• Add a new line at the end of .scss files
• Trim excess whitespace

Formatting

• All selectors are lower case, hyphen separated aka “spinal case” eg. .my-class-name
• Always prefer Sass’s double-slash // commenting, even for block comments
• Avoid specifying units for zero values, e.g. margin: 0; instead of margin: 0px;
• Always add a semicolon to the end of a property/value declaration
• Use leading zeros for decimal values opacity: 0.4; instead of opacity: .4;
• Put spaces before and after child selector div > span instead of div>span

Sass Specifics

Internal order of a .scss file

1. Imports
2. Variables
3. Base Styles
4. Experiment Styles

Example:

//------------------------------
// Modal
//------------------------------

@import "../constants";
@import "../helpers";

$DBmodal-namespace: "c-modal" !default;
$DBmodal-padding: 32px;

$DBmodal-background: #fff !default;
$DBmodal-background-alt: color(gray, x-light) !default;

.o-modal { ... }

// Many lines later...

// EXPERIMENT: experiment-rule-name
.o-modal--experiment { ... }
// END EXPERIMENT: experiment-rule-name

Variables

• Define all variables at the top of the file after the imports
• Namespace local variables with the filename (SASS has no doc level scope)
  • eg business_contact.scss →$business_contact_font_size: 14px;
• Local variables should be $snake_lowercase
• Global constants should be $SNAKE_ALL_CAPS

Color

• Use the defined color constants via the color function
• Lowercase all hex values #fffff
• Limit alpha values to a maximum of two decimal places. Always use a leading zero.

Example:

// Bad
.c-link {
  color: #007ee5;
  border-color: #FFF;
  background-color: rgba(#FFF, .0625);
}

// Good
.c-link {
  color: color(blue);
  border-color: #ffffff;
  background-color: rgba(#ffffff, 0.1);
}

Experiments

Wrap experiment styles with comments:

// EXPERIMENT: experiment-rule-name
.stuff { ... }
// END EXPERIMENT: experiment-rule-name

Rule Ordering

Properties and nested declarations should appear in the following order, with line breaks between groups:

1. Any @ rules
2. Layout and box-model properties

• margin, padding, box-sizing, overflow, position, display, width/height, etc.

3. Typographic properties

• E.g. font-, line-height, letter-spacing, text-, etc.

4. Stylistic properties

• color, background-*, animation, border, etc.

5. UI properties

• appearance, cursor, user-select, pointer-events, etc.

6. Pseudo-elements

• ::after, ::before, ::selection, etc.

7. Pseudo-selectors

• :hover, :focus, :active, etc.

8. Modifier classes
9. Nested elements

Here’s a comprehensive example:

.c-btn {
    @extend %link--plain;

    display: inline-block;
    padding: 6px 12px;

    text-align: center;
    font-weight: 600;

    background-color: color(blue);
    border-radius: 3px;
    color: white;

    &::before {
        content: '';
    }

    &:focus, &:hover {
        box-shadow: 0 0 0 1px color(blue, .3);
    }

    &--big {
        padding: 12px 24px;
    }

    > .c-icon {
        margin-right: 6px;
    }
}

Nesting

• As a general rule of thumb, avoid nesting selectors more than 3 levels deep
• Prefer using nesting as a convenience to extend the parent selector over targeting nested elements. For example:

  .block {
      padding: 24px;
  
      &--mini {
          padding: 12px;
      }
  }

Nesting can be really easily avoided by smart class naming (with the help of BEM) and avoiding bare tag selectors.

BEM

Block: Unique, meaningful names for a logical unit of style. Avoid excessive shorthand.

• Good: .alert-box or .recents-intro or .button
• Bad: .feature or .content or .btn

Element: styles that only apply to children of a block. Elements can also be blocks themselves. Class name is a concatenation of the block name, two underscores and the element name. Examples:

• .alert-box__close
• .expanding-section__section

Modifier: override or extend the base styles of a block or element with modifier styles. Class name is a concatenation of the block (or element) name, two hyphens and the modifier name. Examples:

• .alert-box--success
• .expanding-section--expanded

BEM Best practices

Don't @extend block modifiers with the block base.

• Good: <div class="my-block my-block--modifier">
• Bad: <div class="my-block--modifier">

Don't create elements inside elements. If you find yourself needing this, consider converting your element into a block.

• Bad: .alert-box__close__button

Choose your modifiers wisely. These two rules have very different meaning:

.block--modifier .block__element { color: red; }
.block__element--modifier { color: red; }

Selector Naming

• Try to use BEM-based naming for your class selectors
  • When using modifier classes, always require the base/unmodified class is present
• Use Sass’s nesting to manage BEM selectors like so:

  .block {
      &--modifier { // compiles to .block--modifier
          text-align: center;
      }
  
      &__element { // compiles to .block__element
          color: red;
  
          &--modifier { // compiles to .block__element--modifier
              color: blue;
          }
      }
  }

Namespaced Classes

There are a few reserved namespaces for classes to provide common and globally-available abstractions.

• .o- for CSS objects. Objects are usually common design patterns (like the Flag object). Modifying these classes could have severe knock-on effects.
• .c- for CSS components. Components are designed pieces of UI—think buttons, inputs, modals, and banners.
• .u- for helpers and utilities. Utility classes are usually single-purpose and have high priority. Things like floating elements, trimming margins, etc.
• .is-, .has- for stateful classes, a la SMACSS. Use these classes for temporary, optional, or short-lived states and styles.
• ._ for hacks. Classes with a hack namespace should be used when you need to force a style with !important or increasing specificity, should be temporary, and should not be bound onto.
• .t- for theme classes. Pages with unique styles or overrides for any objects or components should make use of theme classes.

Separation of Concerns (One Thing Well™)

You should always try to spot common code—padding, font sizes, layout patterns—and abstract them to reusable, namespaced classes that can be chained to elements and have a single responsibility. Doing so helps prevent overrides and duplicated rules, and encourages a separation of concerns.

// Bad code
// HTML:
// <div class="modal compact">...</div>
.modal {
    padding: 32px;
    background-color: color(gray, x-light);

    &.compact {
        padding: 24px;
    }
}

// Good code
// HTML:
// <div class="c-modal u-l-island">...</div>
// <div class="c-modal u-l-isle">...</div>

// components/_modal.scss
.c-modal {
    background-color: color(gray, x-light);
}

// helpers/_layout.scss
.u-l-island {
    padding: 32px;
}

.u-l-isle {
    padding: 24px;
}

Media Queries

Media queries should be within the CSS selector as per SMACSS

.selector {
      float: left;

      @media only screen and (max-width: 767px) {
        float: none;
      }
}

Create variables for frequently used breakpoints

$SCREEN_SM_MAX: "max-width: 767px";

.selector {
      float: left;

      @media only screen and ($SCREEN_SM_MAX) {
        float: none;
      }
}