12 CSS Best practices

A best-practices of Cascade Style Sheet (CSS).

Namespaces

Components should always be assigned a unique namespace prefix.

• The namespace can be as short as a single character, or as long as 5 characters
• Where possible, the namespace should be a meaningful shorthand
• In class names, the namespace must be followed by a single dash
• Views should be treated as individual components

Consider the following example, where we assigned ddl to a drop down list component. Take note of the class names.

Good

.ddl-container {
  // ...
}

.ddl-item-list {
  // ...
}

.ddl-item {
  // ...
}

Bad

.item-list {
  // ...
}

.dropdown-item-list {
  // ...
}

.xyz-item-list {
  // ...
}

Classes that are meant to be shared among a large set of elements, or provide reusable styles, should be grouped under a universal namespace, such as uv.

Good

.uv-clearfix {
  // ...
}

Bad

.clearfix {
  // ...
}

See Selectors and Nesting for information in regard to how styles should be overridden

Classes

Class names must follow a few rules.

• Must be all-lowercase
• Words must be separated by single dashes
• As short as possible, but as long as necessary
  • Don’t abbreviate words carelessly
• Name things consistently
• Meaningful description of the elements that should use it
• Keep your non-prefix word count below 4

Good

.ddl-item {
  // ...
}

.ddl-selected {
  // ...
}

.ddl-item-selected {
  // ...
}

Bad

.ddlItem {
  // ...
}

.ddl-item-container-text {
  // ...
}

.ddl-foo-bar-baz {
  // ...
}

Attributes

Attributes make decent selectors from time to time. Some ground rules apply.

• If the “exists” check suffices, use that
• Don’t overqualify using a tag name

Good

[href] {
  // ...
}

Bad

a[href] {
  // ...
}

[href^='http'] {
  // ...
}

id attribute

While the id attribute might be fine in HTML and JavaScript, it should be avoided entirely inside stylesheets. Few reasons.

• ID selectors are not reusable
• Priority nightmares
• “Bad Code”, Dogmatism, etc.

Good

.ur-name {
  // ...
}

Bad

#ur-name {
  // ...
}

Just assign a class name to the element.

Tag Names

Tag names in selectors follow a few rules.

• Application-level styles that are only overridden in a few places are okay to use tag name selectors
• Not semantic. Avoid where possible, use class names instead
• Fine to use when there’s a ton of elements under the same namespace that need a small tweak
• Don’t over qualify (a.foo)

Good

button {
  padding: 5px;
  margin-right: 3px;
}

.ddl-button {
  background-color: #f00;
}

Bad

.ddl-container button {
  background-color: #f00;
}

Selectors and Nesting

Styles shouldn’t need to be nested more than three (four at worst) levels deep. This includes pseudo-selectors. If you find yourself going further, think about re-organizing your rules (either the specificity needed, or the layout of the nesting).

Good

.sg-title-icon:before {
  // ...
}

.dg-container .sg-title {
  font-size: 1.1em; // larger segment title inside dialogs
}

Bad

.dg-container .sg-container .sg-title {
  font-size: 1.1em;
}

.dg-container .sg-title span:before {
  // ...
}

If a component needs to be different within another component, these rules apply.

• Where possible, give a class name using the parent namespace to the child component
• If that’s not possible, then use a nested selector

Suppose you have a User List component .ul-* and a User Card component .uc-*.

Good

<div class='ul-container'>
  <div class='uc-container'>
    <span class='uc-name ul-card-name'>John Doe</span>
  </div>
</div>

.ul-card-name {
  // ...
}

Okay

<div class='ul-container'>
  <div class='uc-container'>
    <span class='uc-name'>John Doe</span>
  </div>
</div>

.ul-container .uc-name {
  // ...
}

Bad

<div class='ul-container'>
  <div class='uc-container'>
    <span class='uc-name uc-name-in-ul'>John Doe</span>
  </div>
</div>

.uc-name-in-ul {
  // ...
}

Organization

Ideally, you should keep your stylesheets separated in different files. Either of the approaches below is fine. The former is prefered.

• Use a single all.{styl,less,scss} file and have it @import every other file
• Use a build tool to glob the styles directory

A few rules apply.

• Each component should take up its own file
• Styles applied globally to tag names, see Tag Names, should be kept in a single file
• Where possible split presentation-specific styles from layout-specific styles. See below

Presentation-Specific vs Layout-Specific Styles

Presentation-Specific styles are those that only alter the visual design of the element, but don’t change its dimensions or position in a meaningful way. The examples below are presentation-specific.

• Rules such as color, font-weight, or font-variant
• Rules that animate other properties
• font-size is not considered a meaningful dimension change
• padding may fit this category (loosely), but only if box-sizing: border-box; is in effect
• max-width and max-height may fit either category, but it’s generally reasonable to consider them presentation-specific

Layout-Specific Styles are those that change the dimensions or positioning of DOM elements. These are mostly layout-specific.

• Rules such as margin or padding
• width, and height
• The element’s position
• z-index, definitely

Where possible, it’s suggested to explicitly split styles into these two categories. The explicit differentiation could be made in a few different ways.

• (bad) No differentiation
• (decent) Layout-specific first, presentation-specific later
• (good) A line-break between both categories
• (better) Split in subsequent style declarations using the same selector
• (best) Declaring the rules in different files altogether

Good

.foo {
  position: fixed;
  top: 8px;
  right: 8px;
  padding: 2px;
  font-weight: bold;
  background-color: #333;
  color: #f00;
}

.foo {
  position: fixed;
  top: 8px;
  right: 8px;
  padding: 2px;

  font-weight: bold;
  background-color: #333;
  color: #f00;
}

.foo {
  position: fixed;
  top: 8px;
  right: 8px;
  padding: 2px;
}

.foo {
  font-weight: bold;
  background-color: #333;
  color: #f00;
}

Bad

.foo {
  font-weight: bold;
  background-color: #333;
  color: #f00;
  position: fixed;
  top: 8px;
  right: 8px;
  padding: 2px;
}

.foo {
  right: 8px;
  color: #f00;
  padding: 2px;
  top: 8px;
  background-color: #333;
  font-weight: bold;
  position: fixed;
}

Styles

These rules apply to your CSS property values

• If the value of a property is 0, do not specify units
• The !important rule should be aggressively avoided.
  • Keep style rules in a sensible order
  • Compose styles to dissipate the need for an !important rule
  • Fine to use in limited cases
    • Overlays
    • Declarations of the display: none !important; type
• Keep z-index levels in variables in a single file. Avoids confusion about what level should be given to an element, and arbitrarily-high 999-style values
• Use hex color codes #000 unless there’s an explicit need for an rgba declaration
• Dislike magic numbers
• Avoid mixing units
• Unit-less line-height is preferred because it does not inherit a percentage value of its parent element, but instead is based on a multiplier of the font-size.

Good

.btn {
  color: #222;
}

.btn-error {
  color: #f00;
}

Bad

.btn-red {
  color: #f00 !important;
}

.btn {
  color: #222;
}

Media Queries

If you are reading this, I salute you. You’re almost as boring as I am. I’m more boring because I actually wrote the damn thing. It’s not a contest, though.

A few rules apply to media queries.

• Settle for a few (2-3) breakpoints and use those only
• Don’t wrap entire stylesheets in media queries
• Instead, modularize media queries wherever possible, keep them relevant to the components
• Approach your styles in a Mobile First manner. Generally you add more things as you get more real estate. Mobile First logically follows

Good

.co-field {
  width: 120px;
}

@media only screen and (min-width: 768px) {
  .co-field {
    width: 400px;
    color: #f00;
  }
}

Bad

.co-field {
  width: 400px;
  color: #f00;
}

@media only screen and (max-width: 768px) {
  .co-field {
    width: 120px;
    color: initial;
  }
}

Frameworks and Vendor Styles

You should shy away from all of these. A few rules apply.

• Stay away from frameworks
• Use normalize.css if you want
• Vendor styles, such as those required by external components are okay, and they should come before you define any of your own styles

Languages

Some rules apply to stylesheet, regardless of language.

• Use a pre-processor language where possible
• Use soft-tabs with a two space indent
• One line per selector
• One (or more) line(s) per property declaration
  • Long, comma-separated property values (such as collections of gradients or shadows) can be arranged across multiple lines in an effort to improve readability and produce more useful diffs.
• Comments that refer to selector blocks should be on a separate line immediately before the block to which they refer
• Use a plugin such as TrailingSpaces in Sublime Text to get rid of trailing spaces

Good

.foo {
  color: #f00;
}

.foo,
.bar,
.baz {
  color: #f00;
}

Bad

.foo {
    color: #f00;
}

.foo, .bar, .baz {
  color: #f00;
}

.foo {
  color: red;
}

Not Stylus

These rules apply to every language except Stylus.

• Always end property declarations with a semicolon
• Put a single space after : in property declarations
• Put spaces before { in rule declarations

Good

.foo {
  color: #f00;
}

Bad

.foo{
  color: #f00;
}

.foo {
  color:#f00;
}

.foo {
  color: #f00
}

Not CSS

Rules applicable to most pre-processor languages.

• Put comments in // statements
• Prefer nested selectors .foo { .bar {} } vs .foo .bar {}
  • Only if both .foo and .foo .bar need styling
• Use &{selector} to concatenate selectors
• Don’t blindly over-nest
• Keep z-index levels in a single file, using variables

Good

// foo

.bar {
  // ...

  .baz {
    ///...
  }
}

Bad

.bar {
  // ...
}

.bar .baz {
  // ...
}

Stylus

Rules specific to Stylus.

• Omit brackets {} in rule declarations
• Omit : and ; in property declarations
• Use transparent mix-ins
• Use nib

Good (Stylus)

// foo

.foo
  color #f00

.foo
  color #f00

  .bar
    padding 2px

Bad (Stylus)

.foo {
  color: #f00;
}

.foo {
  color: #f00;
}

.foo .bar {
  padding: 2px;
}