added css guidelines

.gitignore

@@ -8,3 +8,4 @@
 /docs/_build/
 /.tox/
 /venv
+/node_modules/

.scss-lint.yml

@@ -0,0 +1,154 @@
+
+scss_files: 'wagtail/*/static/**/*.scss'
+exclude: 'wagtail/**/static/**/vendor/*.scss'
+
+linters:
+    BorderZero:
+        enabled: true
+
+    Indentation:
+        severity: warning
+        width: 4
+        allow_non_nested_indentation: true
+        character: space
+
+    ColorKeyword:
+        enabled: true
+
+    ColorVariable:
+        enabled: true
+
+    BangFormat:
+        space_before_bang: true
+        space_after_bang: false
+
+    Comment:
+        enabled: true
+
+    DeclarationOrder:
+        enabled: true
+
+    DuplicateProperty:
+        enabled: false
+
+    ElsePlacement:
+        enabled: true
+
+    EmptyLineBetweenBlocks:
+        enabled: true
+
+    EmptyRule:
+        enabled: true
+
+    FinalNewline:
+        present: true
+
+    HexLength:
+        style: short
+
+    HexNotation:
+        style: lowercase
+
+    HexValidation:
+        enabled: true
+
+    IdSelector:
+        enabled: true
+
+    ImportantRule:
+        enabled: true
+
+    ImportPath:
+        enabled: true
+
+    LeadingZero:
+        style: exclude_zero
+
+    MergeableSelector:
+        enabled: false
+
+    NameFormat:
+        allow_leading_underscore: true
+
+    NestingDepth:
+        max_depth: 4
+
+    SelectorDepth:
+        max_depth: 3
+
+    SelectorFormat:
+        convention: hyphenated_lowercase
+        ignored_names:
+            - js_class
+        ignored_types:
+            - element
+
+    PlaceholderInExtend:
+        enabled: true
+
+    PropertyCount:
+        enabled: false
+
+    QualifyingElement:
+        allow_element_with_attribute: false
+        allow_element_with_class: false
+        allow_element_with_id: false
+
+    Shorthand:
+        enabled: true
+
+    SingleLinePerProperty:
+        enabled: true
+        allow_single_line_rule_sets: true
+
+    SingleLinePerSelector:
+        enabled: true
+
+    SpaceAfterComma:
+        enabled: true
+
+    SpaceAfterPropertyColon:
+        style: at_least_one_space
+
+    SpaceAfterPropertyName:
+        enabled: true
+
+    SpaceBeforeBrace:
+        enabled: true
+        allow_single_line_padding: true
+        style: space
+
+    SpaceBetweenParens:
+        enabled: true
+
+    StringQuotes:
+        style: single_quotes
+
+    TrailingSemicolon:
+        enabled: true
+
+    TrailingZero:
+        enabled: true
+
+    UnnecessaryMantissa:
+        enabled: true
+
+    UnnecessaryParentReference:
+        enbabled: true
+
+    UrlFormat:
+        enabled: true
+
+    UrlQuotes:
+        enabled: true
+
+    VariableForProperty:
+        enabled: false
+
+    VendorPrefix:
+        enabled: false
+
+    ZeroUnit:
+        enabled: true
+
+

docs/reference/css_guidelines.rst

@@ -0,0 +1,151 @@
+Contributing CSS to Wagtail
+===========================
+
+Our CSS is written in Sass, using the SCSS syntax.
+
+Spacing
+~~~~~~~
+
+-  Use soft-tabs with a four space indent. Spaces are the only way to
+   guarantee code renders the same in any person's environment.
+-  Put spaces after ``:`` in property declarations.
+-  Put spaces before ``{`` in rule declarations.
+-  Put line breaks between rulesets.
+-  When grouping selectors, keep individual selectors to a single line.
+-  Place closing braces of declaration blocks on a new line.
+-  Each declaration should appear on its own line for more accurate
+   error reporting.
+-  Add a newline at the end of your ``.scss`` files.
+-  Strip trailing whitespace from your rules.
+
+Formatting
+~~~~~~~~~~
+
+-  Use hex color codes ``#000`` unless using ``rgba()`` in raw CSS
+   (SCSS' ``rgba()`` function is overloaded to accept hex colors as a
+   param, e.g., ``rgba(#000, .5)``).
+-  Use ``//`` for comment blocks (instead of ``/* */``).
+-  Use single quotes for string values
+   ``background: url('my/image.png')`` or ``content: 'moose'``
+-  Avoid specifying units for zero values, e.g., ``margin: 0;`` instead
+   of ``margin: 0px;``.
+-  Strive to limit use of shorthand declarations to instances where you
+   must explicitly set all the available values.
+
+Sass imports
+~~~~~~~~~~~~
+
+Leave off underscores and file extensions in includes:
+
+.. code-block:: scss
+
+    // Bad
+    @import 'components/_widget.scss'
+
+    // Better
+    @import 'components/widget'
+
+Pixels vs. ems
+~~~~~~~~~~~~~~
+
+Use ``rems`` for ``font-size`` with a ``px`` fallback, because it offers
+absolute control over text. Additionally, 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``.
+
+Specificity (classes vs. ids)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Prefer classes over IDs in CSS code. IDs can be overly specific and lead
+to duplication of CSS. When in doubt, use a class name.
+
+When styling a component, start with an element + class namespace,
+prefer direct descendant selectors by default, and use as little
+specificity as possible. Here is a good example:
+
+.. code-block:: html
+
+    <ul class="category-list">
+      <li class="item">Category 1</li>
+      <li class="item">Category 2</li>
+      <li class="item">Category 3</li>
+    </ul>
+
+.. code-block:: scss
+
+    .category-list { // element + class namespace
+
+      // Direct descendant selector > for list items
+      > li {
+        list-style-type: disc;
+      }
+
+      // Minimal specificity for all links
+      a {
+        color: #f00;
+      }
+    }
+
+Class naming conventions
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Never reference ``js-`` prefixed class names from CSS files. ``js-`` are
+used exclusively from JS files.
+
+Use the SMACSS ``is-`` `prefix <https://smacss.com/book/type-state>`__
+for state rules that are shared between CSS and JS.
+
+Misc
+~~~~
+
+As a rule of thumb, avoid unnecessary nesting in SCSS. At most, aim for
+three levels. If you cannot help it, step back and rethink your overall
+strategy (either the specificity needed, or the layout of the nesting).
+
+Examples
+~~~~~~~~
+
+Here are some good examples that apply the above guidelines:
+
+.. code-block:: scss
+
+    // Example of good basic formatting practices
+    .styleguide-format {
+      color: #000;
+      background-color: rgba(0, 0, 0, .5);
+      border: 1px solid #0f0;
+    }
+
+    // Example of individual selectors getting their own lines (for error reporting)
+    .multiple,
+    .classes,
+    .get-new-lines {
+      display: block;
+    }
+
+    // Avoid unnecessary shorthand declarations
+    .not-so-good {
+      margin: 0 0 20px;
+    }
+    .good {
+      margin-bottom: 20px;
+    }
+
+Vendor prefixes
+~~~~~~~~~~~~~~~
+
+Where possible, use the autoprefixer
+
+Linting SCSS
+~~~~~~~~~~~~
+
+The guidelines are included in a ``.scss-lint.yml`` file so that you can
+check that your code conforms to the style guide.
+
+Run the linter from the wagtail project root with ``scss-lint .``.
+You'll need to have the linter installed to do this. You can get it by
+running
+
+.. code-block:: bash
+
+    gem install scss-lint

docs/reference/index.rst

@@ -9,3 +9,4 @@ Reference
     hooks
     project_template
     page
+    css_guidelines