notes/node_modules/stylelint/lib/utils/isStandardSyntaxDeclaration.js

/* @flow */
"use strict"

/**
 * Check whether a declaration is standard
 */
module.exports = function (decl/*: Object*/)/*: boolean*/ {
  const prop = decl.prop,
    parent = decl.parent

  // Declarations belong in a declaration block

  if (parent.type === "root") {
    return false
  }

  // SCSS var (e.g. $var: x), nested list (e.g. $list: (x)) or nested map (e.g. $map: (key:value))
  if (prop[0] === "$") {
    return false
  }

  // Less var (e.g. @var: x), but exclude variable interpolation (e.g. @{var})
  if (prop[0] === "@" && prop[1] !== "{") {
    return false
  }

  // SCSS nested properties (e.g. border: { style: solid; color: red; })
  if (parent.selector && parent.selector[parent.selector.length - 1] === ":" && parent.selector.substring(0, 2) !== "--") {
    return false
  }

  return true
}