node_modules/stylelint/lib/utils/isStandardSyntaxDeclaration.js

/* @flow */
"use strict";

function isStandardSyntaxLang(lang) {
  return (
    lang &&
    (lang === "css" ||
      lang === "custom-template" ||
      lang === "template-literal")
  );
}

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

  // Declarations belong in a declaration block or standard CSS source
  if (parent.type === "root" && !isStandardSyntaxLang(parent.source.lang)) {
    return false;
  }

  // Sass 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;
  }

  // Sass 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;
  }

  // Less &:extend
  if (decl.extend) {
    return false;
  }

  return true;
};