Adds initial styleguide content.

Author: Mike Fowler

.scss-lint.yml

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+linters:
+
+  BorderZero:
+    enabled: true
+    convention: none
+
+  BemDepth:
+    enabled: true
+
+  DeclarationOrder:
+    enabled: false
+
+  LeadingZero:
+    enabled: false
+
+  PropertySortOrder:
+    enabled: false
+
+  QualifyingElement:
+    enabled: false
+
+  SelectorFormat:
+    enabled: true
+    convention: hyphenated_BEM
+    class_convention: ^(?!js-).*
+    class_convention_explanation: should not be written in the form js-*
+
+  SingleLinePerProperty:
+    enabled: true
+    allow_single_line_rule_sets: false
+
+  StringQuotes:
+    enabled: true
+    style: double_quotes

README.md

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+# Airbnb CSS / Sass Styleguide
+
+*A mostly reasonable approach to CSS and Sass*
+
+## Table of Contents
+
+  1. [Terminology](#terminology)
+    - [Rule Declaration](#rule-declaration)
+    - [Selectors](#selectors)
+    - [Properties](#properties)
+  1. [CSS](#css)
+    - [Formatting](#formatting)
+    - [Comments](#comments)
+    - [OOCSS and BEM](#oocss-and-bem)
+    - [ID Selectors](#id-selectors)
+    - [JavaScript hooks](#javascript-hooks)
+  1. [Sass](#sass)
+    - [Syntax](#syntax)
+    - [Ordering](#ordering-of-property-declarations)
+    - [Mixins](#mixins)
+    - [Placeholders](#placeholders)
+    - [Nested selectors](#nested-selectors)
+
+## Terminology
+
+### Rule declaration
+
+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:
+
+```css
+.listing {
+  font-size: 18px;
+  line-height: 1.2;
+}
+```
+
+### Selectors
+
+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:
+
+```css
+.my-element-class {
+  /* ... */
+}
+
+[aria-hidden] {
+  /* ... */
+}
+```
+
+### Properties
+
+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:
+
+```css
+/* some selector */ {
+  background: #f1f1f1;
+  color: #333;
+}
+```
+
+## CSS
+
+### Formatting
+
+* Use soft tabs (2 spaces) for indentation
+* Prefer dashes over camelCasing in class names. Underscores are OK if you're using BEM (see [OOCSS and BEM](#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 `{` in rule declarations
+* Put blank lines between rule declarations
+* In property declarations, put a space after, but not before, the `:` character.
+* Order property declarations alphabetically for consistency
+* Put closing braces of declaration blocks on a new line
+
+**Bad**
+
+```css
+.avatar{
+    border-radius:50%;
+    border:2px solid white; }
+.no, .nope, .not_good {
+    // ...
+}
+#lol-no {
+  // ...
+}
+```
+
+**Good**
+
+```css
+.avatar {
+  border-radius: 50%;
+  border: 2px solid white;
+}
+
+.one,
+.selector,
+.per-line {
+  // ...
+}
+```
+
+### Comments
+
+* 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
+
+### OOCSS and BEM
+
+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”: reusuable, repeatable snippets that can be used independently throughout a website.
+
+  * Nicole Sullivan's [OOCSS wiki](https://github.com/stubbornella/oocss/wiki)
+  * Smashing Magazine's [Introduction to OOCSS](http://www.smashingmagazine.com/2011/12/12/an-introduction-to-object-oriented-css-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](https://css-tricks.com/bem-101/)
+  * Harry Roberts' [introduction to BEM](http://csswizardry.com/2013/01/mindbemding-getting-your-head-round-bem-syntax/)
+
+**Example**
+
+```html
+<article class="listing-card listing-card--featured">
+
+  <h1 class="listing-card__title">Adorable 2BR in the sunny Mission</h1>
+
+  <div class="listing-card__content">
+    <p>Vestibulum id ligula porta felis euismod semper.</p>
+  </div>
+
+</article>
+```
+
+```css
+.listing-card { }
+.listing-card--featured { }
+.listing-card__title { }
+.listing-card__content { }
+```
+
+  * `.listing-card` is the “block” and represents the higher-level component
+  * `.listing-card__title` is an “element” and represents a descendant of `.listing-card` that helps compose the block as a whole.
+  * `.listing-card--featured` is a “modifier” and represents a different state or variation on the `.listing-card` block.
+
+### ID selectors
+
+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](https://developer.mozilla.org/en-US/docs/Web/CSS/Specificity) to your rule declarations.
+
+For more on this subject, read [CSS Wizardry's article](http://csswizardry.com/2014/07/hacks-for-dealing-with-specificity/) on dealing with specificity.
+
+### JavaScript hooks
+
+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-`:
+
+```html
+<button class="btn btn-primary js-request-to-book">Request to Book</button>
+```
+
+## Sass
+
+### Syntax
+
+* Use the .scss syntax, never the original .sass syntax
+* Order your `@extend`, regular CSS and `@include` declarations logically (see below)
+
+### Ordering of property declarations
+
+1. `@extend` declarations
+
+    Just as in other OOP languages, it's helpful to know right away that this “class” inherits from another.
+
+    ```scss
+    .btn-green {
+      @extend %btn;
+      // ...
+    }
+    ```
+
+2. Property declarations
+
+    Now list all standard property declarations, anything that isn't an `@extend`, `@include`, or a nested selector.
+
+    ```scss
+    .btn-green {
+      @extend %btn;
+      background: green;
+      font-weight: bold;
+      // ...
+    }
+    ```
+
+3. `@include` declarations
+
+    Grouping `@include`s at the end makes it easier to read the entire selector, and it also visually separates them from `@extend`s.
+
+    ```scss
+    .btn-green {
+      @extend %btn;
+      background: green;
+      font-weight: bold;
+      @include transition(background 0.5s ease);
+      // ...
+    }
+    ```
+
+4. 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.
+
+    ```scss
+    .btn {
+      @extend %btn;
+      background: green;
+      font-weight: bold;
+      @include transition(background 0.5s ease);
+      
+      .icon {
+        margin-right: 10px;
+      }
+    }
+    ```
+
+### Mixins
+
+Mixins, defined via `@mixin` and called with `@include`, should be used sparingly and only when function arguments are necessary. A mixin without function arguments (i.e. `@mixin hide { display: none; }`) is better accomplished using a placeholder selector (see below) in order to prevent code duplication.
+
+### Placeholders
+
+Placeholders in Sass, defined via `%selector` and used with `@extend`, are a way of defining a block of properties that aren't automatically output in your compiled stylesheet. Instead, other selectors “inherit” from the placeholder, and the relevant selectors are copied to the point in the stylesheet where the placeholder is defined. This is best illustrated with the example below.
+
+Placeholders are powerful but easy to abuse, especially when combined with nested selectors. *As a rule of thumb, avoid creating placeholders with nested declaration blocks, or calling `@extend` inside nested selectors.** Placeholders are great for simple inheritance, but can easily result in the accidental creation of additional selectors without paying close attention to how and where they are used.
+
+**Sass**
+
+```sass
+// Unless we call `@extend %icon` these properties won't be compiled!
+%icon {
+  font-family: 'Airglyphs';
+}
+
+.icon-error {
+  @extend %icon;
+  color: red;
+}
+
+.icon-success {
+  @extend %hide;
+  color: green;
+}
+```
+
+**CSS**
+
+```css
+.icon-error,
+.icon-success {
+  font-family: 'Airglyphs';
+}
+
+.icon-error {
+  color: red;
+}
+
+.icon-success {
+  color: green;
+}
+```
+
+### Nested selectors
+
+**Do not nest selectors more than three levels deep!**
+
+```scss
+.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're writing well thought-out CSS, you should rarely, *if ever* need to do this.