report invalid selector in :not() as a warning

to prevent failures when updating usercss styles that have to override [currently] invalid :not() selectors used by the site

see #356

edit/lint-defaults-csslint.js

@@ -10,6 +10,7 @@ window.linterConfig.defaults.csslint = {
   'duplicate-properties': 1,
   'empty-rules': 1,
   'errors': 1,
+  'warnings': 1,
   'known-properties': 1,
 
   // Default disabled

edit/lint.js

@@ -59,7 +59,7 @@ var linterConfig = {
         // always use default syntax because we don't expose it in config UI
         config.syntax = this.defaults.stylelint.syntax;
       }
-      return config;
+      return Object.assign({}, this.defaults[linter] || {}, config);
     } else {
       return deepCopy(this.defaults[linter] || {});
     }

vendor-overwrites/csslint/csslint.js

@@ -857,6 +857,19 @@ CSSLint.addRule({
   },
 });
 
+CSSLint.addRule({
+  id:       'warnings',
+  name:     'Parsing warnings',
+  desc:     'This rule looks for parser warnings.',
+  browsers: 'All',
+
+  init(parser, reporter) {
+    parser.addListener('warning', ({message, line, col}) => {
+      reporter.report(message, line, col, this);
+    });
+  },
+});
+
 CSSLint.addRule({
   id:       'fallback-colors',
   name:     'Require fallback colors',

vendor-overwrites/csslint/parserlib.js

@@ -4627,7 +4627,7 @@ self.parserlib = (() => {
           parts[0].modifiers.length + (parts[0].elementName ? 1 : 0) > 1 ||
           /^:not\b/i.test(parts[0])) {
         this.fire({
-          type: 'error',
+          type: 'warning',
           message: `Simple selector expected, but found '${arg.join(', ')}'`,
         }, arg[0]);
       }