diff --git a/.editorconfig b/.editorconfig
new file mode 100644
index 0000000..e06d798
--- /dev/null
+++ b/.editorconfig
@@ -0,0 +1,15 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_style = tab
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.md]
+trim_trailing_whitespace = false
+
+[*.{json,md,yml}]
+indent_size = 2
+indent_style = space
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..995098a
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,10 @@
+node_modules
+package-lock.json
+.*
+!.appveyor.yml
+!.editorconfig
+!.gitignore
+!.tape.js
+!.travis.yml
+*.log*
+*.result.css
diff --git a/.tape.js b/.tape.js
new file mode 100644
index 0000000..ee74b99
--- /dev/null
+++ b/.tape.js
@@ -0,0 +1,54 @@
+module.exports = {
+	'postcss-extend-rule': {
+		'basic': {
+			message: 'supports basic usage'
+		},
+		'advanced': {
+			message: 'supports advanced usage (with postcss-nesting)',
+			plugin: () => require('postcss')(
+				require('postcss-nesting'),
+				require('.')
+			)
+		},
+		'errors': {
+			message: 'manages error-ridden usage'
+		},
+		'errors:ignore': {
+			message: 'manages error-ridden usage with { onFunctionalSelector: "ignore", onRecursiveExtend: "ignore", onUnusedExtend: "ignore" } options',
+			options: {
+				onFunctionalSelector: 'ignore',
+				onRecursiveExtend: 'ignore',
+				onUnusedExtend: 'ignore'
+			}
+		},
+		'errors:warn': {
+			message: 'manages error-ridden usage with { onFunctionalSelector: "warn", onRecursiveExtend: "warn", onUnusedExtend: "warn" } options',
+			options: {
+				onFunctionalSelector: 'warn',
+				onRecursiveExtend: 'warn',
+				onUnusedExtend: 'warn'
+			},
+			warning: 6
+		},
+		'errors:throw': {
+			message: 'manages error-ridden usage with { onFunctionalSelector: "throw", onRecursiveExtend: "throw", onUnusedExtend: "throw" } options',
+			options: {
+				onFunctionalSelector: 'throw',
+				onRecursiveExtend: 'throw',
+				onUnusedExtend: 'throw'
+			},
+			error: {
+				reason: 'Unused extend at-rule "some-non-existent-selector"'
+			}
+		},
+		'errors:throw-on-functional-selectors': {
+			message: 'manages error-ridden usage with { onFunctionalSelector: "throw" } options',
+			options: {
+				onFunctionalSelector: 'throw'
+			},
+			error: {
+				reason: 'Encountered functional selector "%test-placeholder"'
+			}
+		}
+	}
+};
diff --git a/.travis.yml b/.travis.yml
new file mode 100644
index 0000000..c564664
--- /dev/null
+++ b/.travis.yml
@@ -0,0 +1,9 @@
+# https://docs.travis-ci.com/user/travis-lint
+
+language: node_js
+
+node_js:
+  - 4
+
+install:
+  - npm install --ignore-scripts
diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..580916a
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,5 @@
+# Changes to PostCSS Extend Rule
+
+### 1.0.0 (September 15, 2017)
+
+- Initial version
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644
index 0000000..00eda4b
--- /dev/null
+++ b/CONTRIBUTING.md
@@ -0,0 +1,65 @@
+# Contributing to PostCSS Extend Rule
+
+You want to help? You rock! Now, take a moment to be sure your contributions
+make sense to everyone else.
+
+## Reporting Issues
+
+Found a problem? Want a new feature?
+
+- See if your issue or idea has [already been reported].
+- Provide a [reduced test case] or a [live example].
+
+Remember, a bug is a _demonstrable problem_ caused by _our_ code.
+
+## Submitting Pull Requests
+
+Pull requests are the greatest contributions, so be sure they are focused in
+scope and avoid unrelated commits.
+
+1. To begin; [fork this project], clone your fork, and add our upstream.
+   ```bash
+   # Clone your fork of the repo into the current directory
+   git clone git@github.com:YOUR_USER/postcss-extend-rule.git
+
+   # Navigate to the newly cloned directory
+   cd postcss-extend-rule
+
+   # Assign the original repo to a remote called "upstream"
+   git remote add upstream git@github.com:jonathantneal/postcss-extend-rule.git
+
+   # Install the tools necessary for testing
+   npm install
+   ```
+
+2. Create a branch for your feature or fix:
+   ```bash
+   # Move into a new branch for your feature
+   git checkout -b feature/thing
+   ```
+   ```bash
+   # Move into a new branch for your fix
+   git checkout -b fix/something
+   ```
+
+3. If your code follows our practices, then push your feature branch:
+   ```bash
+   # Test current code
+   npm test
+   ```
+   ```bash
+   # Push the branch for your new feature
+   git push origin feature/thing
+   ```
+   ```bash
+   # Or, push the branch for your update
+   git push origin update/something
+   ```
+
+That’s it! Now [open a pull request] with a clear title and description.
+
+[already been reported]: issues
+[fork this project]:     fork
+[live example]:          https://codepen.io/pen
+[open a pull request]:   https://help.github.com/articles/using-pull-requests/
+[reduced test case]:     https://css-tricks.com/reduced-test-cases/
diff --git a/LICENSE.md b/LICENSE.md
new file mode 100644
index 0000000..b5bc55c
--- /dev/null
+++ b/LICENSE.md
@@ -0,0 +1,106 @@
+# CC0 1.0 Universal
+
+## Statement of Purpose
+
+The laws of most jurisdictions throughout the world automatically confer
+exclusive Copyright and Related Rights (defined below) upon the creator and
+subsequent owner(s) (each and all, an “owner”) of an original work of
+authorship and/or a database (each, a “Work”).
+
+Certain owners wish to permanently relinquish those rights to a Work for the
+purpose of contributing to a commons of creative, cultural and scientific works
+(“Commons”) that the public can reliably and without fear of later claims of
+infringement build upon, modify, incorporate in other works, reuse and
+redistribute as freely as possible in any form whatsoever and for any purposes,
+including without limitation commercial purposes. These owners may contribute
+to the Commons to promote the ideal of a free culture and the further
+production of creative, cultural and scientific works, or to gain reputation or
+greater distribution for their Work in part through the use and efforts of
+others.
+
+For these and/or other purposes and motivations, and without any expectation of
+additional consideration or compensation, the person associating CC0 with a
+Work (the “Affirmer”), to the extent that he or she is an owner of Copyright
+and Related Rights in the Work, voluntarily elects to apply CC0 to the Work and
+publicly distribute the Work under its terms, with knowledge of his or her
+Copyright and Related Rights in the Work and the meaning and intended legal
+effect of CC0 on those rights.
+
+1. Copyright and Related Rights. A Work made available under CC0 may be
+	 protected by copyright and related or neighboring rights (“Copyright and
+	 Related Rights”). Copyright and Related Rights include, but are not limited
+	 to, the following:
+	1. the right to reproduce, adapt, distribute, perform, display,
+		 communicate, and translate a Work;
+	2. moral rights retained by the original author(s) and/or performer(s);
+	3. publicity and privacy rights pertaining to a person’s image or likeness
+		 depicted in a Work;
+	4. rights protecting against unfair competition in regards to a Work,
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+		 including any amended or successor version of such directive); and
+	7. other similar, equivalent or corresponding rights throughout the world
+		 based on applicable law or treaty, and any national implementations
+		 thereof.
+
+2. Waiver. To the greatest extent permitted by, but not in contravention of,
+applicable law, Affirmer hereby overtly, fully, permanently, irrevocably and
+unconditionally waives, abandons, and surrenders all of Affirmer’s Copyright
+and Related Rights and associated claims and causes of action, whether now
+known or unknown (including existing as well as future claims and causes of
+action), in the Work (i) in all territories worldwide, (ii) for the maximum
+duration provided by applicable law or treaty (including future time
+extensions), (iii) in any current or future medium and for any number of
+copies, and (iv) for any purpose whatsoever, including without limitation
+commercial, advertising or promotional purposes (the “Waiver”). Affirmer makes
+the Waiver for the benefit of each member of the public at large and to the
+detriment of Affirmer’s heirs and successors, fully intending that such Waiver
+shall not be subject to revocation, rescission, cancellation, termination, or
+any other legal or equitable action to disrupt the quiet enjoyment of the Work
+by the public as contemplated by Affirmer’s express Statement of Purpose.
+
+3. Public License Fallback. Should any part of the Waiver for any reason be
+judged legally invalid or ineffective under applicable law, then the Waiver
+shall be preserved to the maximum extent permitted taking into account
+Affirmer’s express Statement of Purpose. In addition, to the extent the Waiver
+is so judged Affirmer hereby grants to each affected person a royalty-free, non
+transferable, non sublicensable, non exclusive, irrevocable and unconditional
+license to exercise Affirmer’s Copyright and Related Rights in the Work (i) in
+all territories worldwide, (ii) for the maximum duration provided by applicable
+law or treaty (including future time extensions), (iii) in any current or
+future medium and for any number of copies, and (iv) for any purpose
+whatsoever, including without limitation commercial, advertising or promotional
+purposes (the “License”). The License shall be deemed effective as of the date
+CC0 was applied by Affirmer to the Work. Should any part of the License for any
+reason be judged legally invalid or ineffective under applicable law, such
+partial invalidity or ineffectiveness shall not invalidate the remainder of the
+License, and in such case Affirmer hereby affirms that he or she will not (i)
+exercise any of his or her remaining Copyright and Related Rights in the Work
+or (ii) assert any associated claims and causes of action with respect to the
+Work, in either case contrary to Affirmer’s express Statement of Purpose.
+
+4. Limitations and Disclaimers.
+		1. No trademark or patent rights held by Affirmer are waived, abandoned,
+			 surrendered, licensed or otherwise affected by this document.
+		2. Affirmer offers the Work as-is and makes no representations or
+			 warranties of any kind concerning the Work, express, implied, statutory
+			 or otherwise, including without limitation warranties of title,
+			 merchantability, fitness for a particular purpose, non infringement, or
+			 the absence of latent or other defects, accuracy, or the present or
+			 absence of errors, whether or not discoverable, all to the greatest
+			 extent permissible under applicable law.
+		3. Affirmer disclaims responsibility for clearing rights of other persons
+			 that may apply to the Work or any use thereof, including without
+			 limitation any person’s Copyright and Related Rights in the Work.
+			 Further, Affirmer disclaims responsibility for obtaining any necessary
+			 consents, permissions or other rights required for any use of the Work.
+		4. Affirmer understands and acknowledges that Creative Commons is not a
+			 party to this document and has no duty or obligation with respect to
+			 this CC0 or use of the Work.
+
+For more information, please see
+https://creativecommons.org/publicdomain/zero/1.0/.
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..542a976
--- /dev/null
+++ b/README.md
@@ -0,0 +1,185 @@
+# PostCSS Extend Rule [<img src="https://postcss.github.io/postcss/logo.svg" alt="PostCSS Logo" width="90" height="90" align="right">][postcss]
+
+[![NPM Version][npm-img]][npm-url]
+[![Build Status][cli-img]][cli-url]
+[![Gitter Chat][git-img]][git-url]
+
+[PostCSS Extend Rule] lets you use the `@extend` at-rule and
+[Functional Selectors] in CSS, following the speculative
+[CSS Extend Rules Specification].
+
+```css
+%thick-border {
+  border: thick dotted red;
+}
+
+.serious-modal {
+  font-style: normal;
+  font-weight: bold;
+
+  @media (max-width: 240px) {
+    @extend .modal:hover;
+  }
+}
+
+.modal {
+  @extend %thick-border;
+
+  color: red;
+}
+
+.modal:hover:not(:focus) {
+  outline: none;
+}
+
+/* becomes */
+
+.serious-modal {
+  font-style: normal;
+  font-weight: bold;
+}
+
+@media (max-width: 240px) {
+  .serious-modal:not(:focus) {
+    outline: none;
+  }
+}
+
+.modal {
+  border: thick dotted red;
+  color: red;
+}
+
+.modal:hover:not(:focus) {
+  outline: none;
+}
+```
+
+## Usage
+
+Add [PostCSS Extend Rule] to your build tool:
+
+```bash
+npm install postcss-extend-rule --save-dev
+```
+
+#### Node
+
+Use [PostCSS Extend Rule] to process your CSS:
+
+```js
+require('postcss-extend-rule').process(YOUR_CSS /*, PostCSS Options, Options */);
+```
+
+#### PostCSS
+
+Add [PostCSS] to your build tool:
+
+```bash
+npm install postcss --save-dev
+```
+
+Use [PostCSS Extend Rule] as a plugin:
+
+```js
+postcss([
+  require('postcss-extend-rule')(/* Options */)
+]).process(YOUR_CSS);
+```
+
+#### Gulp
+
+Add [Gulp PostCSS] to your build tool:
+
+```bash
+npm install gulp-postcss --save-dev
+```
+
+Use [PostCSS Extend Rule] in your Gulpfile:
+
+```js
+var postcss = require('gulp-postcss');
+
+gulp.task('css', function () {
+  return gulp.src('./src/*.css').pipe(
+    postcss([
+      require('postcss-extend-rule')(/* Options */)
+    ])
+  ).pipe(
+    gulp.dest('.')
+  );
+});
+```
+
+#### Grunt
+
+Add [Grunt PostCSS] to your build tool:
+
+```bash
+npm install grunt-postcss --save-dev
+```
+
+Use [PostCSS Extend Rule] in your Gruntfile:
+
+```js
+grunt.loadNpmTasks('grunt-postcss');
+
+grunt.initConfig({
+  postcss: {
+    options: {
+      use: [
+        require('postcss-extend-rule')(/* Options */)
+      ]
+    },
+    dist: {
+      src: '*.css'
+    }
+  }
+});
+```
+
+## Options
+
+### onFunctionalSelector
+
+The `onFunctionalSelector` option determines how functional selectors should be
+handled. Its options are:
+
+- `remove` (default) removes any functional selector
+- `ignore` ignores any functional selector and moves on
+- `warn` warns the user whenever it encounters a functional selector
+- `throw` throws an error if ever it encounters a functional selector
+
+### onRecursiveExtend
+
+The `onRecursiveExtend` option determines how recursive extend at-rules should
+be handled. Its options are:
+
+- `remove` (default) removes any recursive extend at-rules
+- `ignore` ignores any recursive extend at-rules and moves on
+- `warn` warns the user whenever it encounters a recursive extend at-rules
+- `throw` throws an error if ever it encounters a recursive extend at-rules
+
+### onUnusedExtend
+
+The `onUnusedExtend` option determines how an unused extend at-rule should be
+handled. Its options are:
+
+- `remove` (default) removes any unused extend at-rule
+- `ignore` ignores any unused extend at-rule and moves on
+- `warn` warns the user whenever it encounters an unused extend at-rule
+- `throw` throws an error if ever it encounters an unused extend at-rule
+
+[npm-url]: https://www.npmjs.com/package/postcss-extend-rule
+[npm-img]: https://img.shields.io/npm/v/postcss-extend-rule.svg
+[cli-url]: https://travis-ci.org/jonathantneal/postcss-extend-rule
+[cli-img]: https://img.shields.io/travis/jonathantneal/postcss-extend-rule.svg
+[git-url]: https://gitter.im/postcss/postcss
+[git-img]: https://img.shields.io/badge/chat-gitter-blue.svg
+
+[CSS Extend Rules Specification]: https://jonathantneal.github.io/specs/css-extend-rule/
+[Functional Selectors]: https://jonathantneal.github.io/specs/css-extend-rule/#functional-selector
+[Gulp PostCSS]: https://github.com/postcss/gulp-postcss
+[Grunt PostCSS]: https://github.com/nDmitry/grunt-postcss
+[PostCSS]: https://github.com/postcss/postcss
+[PostCSS Extend Rule]: https://github.com/jonathantneal/postcss-extend-rule
diff --git a/index.js b/index.js
new file mode 100644
index 0000000..cf600ea
--- /dev/null
+++ b/index.js
@@ -0,0 +1,119 @@
+// external tooling
+const postcss                = require('postcss');
+const transformNestingAtRule = require('postcss-nesting/lib/transform-nesting-atrule');
+
+// extend at-rule match
+const extendMatch = /^(extend)$/i;
+
+// functional selector match
+const functionalSelectorMatch = /(^|[^\w-])(%[_a-zA-Z]+[_a-zA-Z0-9-]*)([^\w-]|$)/i;
+
+// plugin
+module.exports = postcss.plugin('postcss-extend-rule', (rawopts) => {
+	// options ( onFunctionalSelector, onRecursiveExtend, onUnusedExtend)
+	const opts = Object(rawopts);
+
+	return (root, result) => {
+		// for each extend at-rule
+		root.walkAtRules(extendMatch, (extendAtRule) => {
+			// do not revisit visited extend at-rules
+			if (!extendAtRule.__extendAtRuleVisited) {
+				extendAtRule.__extendAtRuleVisited = true;
+
+				// selector identifier
+				const selectorIdMatch = getSelectorIdMatch(extendAtRule.params);
+
+				// extending rules
+				const extendingRules = getExtendingRules(selectorIdMatch, extendAtRule);
+
+				// if there are extending rules
+				if (extendingRules.length) {
+					// replace the extend at-rule with the extending rules
+					extendAtRule.replaceWith(extendingRules);
+
+					// transform these nesting at-rules
+					extendingRules.forEach(transformNestingAtRule);
+				} else {
+					// manage unused extend at-rules
+					const unusedExtendMessage = `Unused extend at-rule "${extendAtRule.params}"`;
+
+					if (opts.onUnusedExtend === 'throw') {
+						throw extendAtRule.error(unusedExtendMessage, { word: extendAtRule.name });
+					} else if (opts.onUnusedExtend === 'warn') {
+						extendAtRule.warn(result, unusedExtendMessage);
+					} else if (opts.onUnusedExtend !== 'ignore') {
+						extendAtRule.remove();
+					}
+				}
+			} else {
+				// manage revisited extend at-rules
+				const revisitedExtendMessage = `Revisited extend at-rule "${extendAtRule.params}"`;
+
+				if (opts.onRecursiveExtend === 'throw') {
+					throw extendAtRule.error(revisitedExtendMessage, { word: extendAtRule.name });
+				} else if (opts.onRecursiveExtend === 'warn') {
+					extendAtRule.warn(result, revisitedExtendMessage);
+				} else if (opts.onRecursiveExtend !== 'ignore') {
+					extendAtRule.remove();
+				}
+			}
+		});
+
+		root.walkRules(functionalSelectorMatch, (functionalRule) => {
+			// manage encountered functional selectors
+			const functionalSelectorMessage = `Encountered functional selector "${functionalRule.selector}"`;
+
+			if (opts.onFunctionalSelector === 'throw') {
+				throw functionalRule.error(functionalSelectorMessage, { word: functionalRule.selector.match(functionalSelectorMatch)[1] });
+			} else if (opts.onFunctionalSelector === 'warn') {
+				functionalRule.warn(result, functionalSelectorMessage);
+			} else if (opts.onFunctionalSelector !== 'ignore') {
+				functionalRule.remove();
+			}
+		});
+	};
+});
+
+function getExtendingRules(selectorIdMatch, extendAtRule) {
+	// extending rules
+	const extendingRules = [];
+
+	// for each rule found from root of the extend at-rule with a matching selector identifier
+	extendAtRule.root().walkRules(selectorIdMatch, (matchingRule) => {
+		// nesting selectors for the selectors matching the selector identifier
+		const nestingSelectors = matchingRule.selectors.filter(
+			(selector) => selectorIdMatch.test(selector)
+		).map(
+			(selector) => selector.replace(selectorIdMatch, '$1&$3')
+		).join(',');
+
+		// matching rule’s cloned nodes
+		const nestingNodes = matchingRule.clone().nodes;
+
+		// push the matching rule to the extending rules
+		extendingRules.push(
+			extendAtRule.clone({
+				name: 'nest',
+				params: nestingSelectors,
+				nodes: nestingNodes,
+				// empty the extending rules, as they are likely non-comforming
+				raws: {}
+			})
+		);
+	});
+
+	// return the extending rules
+	return extendingRules;
+}
+
+function getSelectorIdMatch(selectorIds) {
+	// escape the contents of the selector id to avoid being parsed as regex
+	const escapedSelectorIds = postcss.list.comma(selectorIds).map(
+		(selectorId) => selectorId.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')
+	).join('|');
+
+	// selector unattached to an existing selector
+	const selectorIdMatch = new RegExp(`(^|[^\\w-])(${escapedSelectorIds})([^\\w-]|$)`, '');
+
+	return selectorIdMatch;
+}
diff --git a/package.json b/package.json
new file mode 100644
index 0000000..41d5956
--- /dev/null
+++ b/package.json
@@ -0,0 +1,58 @@
+{
+  "name": "postcss-extend-rule",
+  "version": "1.0.0",
+  "description": "Use the @extend at-rule and functional selectors in CSS",
+  "author": "Jonathan Neal <jonathantneal@hotmail.com>",
+  "license": "CC0-1.0",
+  "repository": "jonathantneal/postcss-extend-rule",
+  "homepage": "https://github.com/jonathantneal/postcss-extend-rule#readme",
+  "bugs": "https://github.com/jonathantneal/postcss-extend-rule/issues",
+  "main": "index.js",
+  "files": [
+    "index.js"
+  ],
+  "scripts": {
+    "clean": "git clean -X -d -f",
+    "prepublish": "npm test",
+    "test": "echo 'Running tests...'; npm run test:js && npm run test:tape",
+    "test:js": "eslint *.js --cache --ignore-pattern .gitignore",
+    "test:tape": "postcss-tape"
+  },
+  "engines": {
+    "node": ">=4.0.0"
+  },
+  "dependencies": {
+    "postcss": "^6.0.11",
+    "postcss-nesting": "^4.2.0"
+  },
+  "devDependencies": {
+    "eslint": "^4.7.0",
+    "eslint-config-dev": "2.0.0",
+    "postcss-tape": "2.1.0",
+    "pre-commit": "^1.2.2"
+  },
+  "eslintConfig": {
+    "extends": "dev"
+  },
+  "keywords": [
+    "postcss",
+    "css",
+    "postcss-plugin",
+    "extend",
+    "matched",
+    "matches",
+    "match",
+    "selectors",
+    "subclassing",
+    "subclasses",
+    "subclass",
+    "styling",
+    "styles",
+    "style",
+    "placeholder",
+    "placehold",
+    "selectors",
+    "selector",
+    "chaining"
+  ]
+}
diff --git a/test/advanced.css b/test/advanced.css
new file mode 100644
index 0000000..48b678d
--- /dev/null
+++ b/test/advanced.css
@@ -0,0 +1,24 @@
+%thick-border {
+	border: thick dotted red;
+}
+
+.serious-modal {
+	font-style: normal;
+	font-weight: bold;
+
+	@media (max-width: 240px) {
+		@extend .modal:hover;
+	}
+}
+
+.modal {
+	@extend %thick-border;
+
+	color: red;
+
+	&:hover {
+		&:not(:focus) {
+			outline: none;
+		}
+	}
+}
diff --git a/test/advanced.expect.css b/test/advanced.expect.css
new file mode 100644
index 0000000..db86fca
--- /dev/null
+++ b/test/advanced.expect.css
@@ -0,0 +1,20 @@
+.serious-modal {
+	font-style: normal;
+	font-weight: bold;
+}
+
+@media (max-width: 240px) {
+
+	.serious-modal:not(:focus) {
+		outline: none;
+	}
+}
+
+.modal {
+	border: thick dotted red;
+	color: red;
+}
+
+.modal:hover:not(:focus) {
+	outline: none;
+}
diff --git a/test/basic.css b/test/basic.css
new file mode 100644
index 0000000..d533d1a
--- /dev/null
+++ b/test/basic.css
@@ -0,0 +1,14 @@
+.modal {
+	border: thick dotted red;
+	color: red;
+}
+
+.modal:hover {
+	outline: none;
+}
+
+.serious-modal {
+	@extend .modal;
+
+	font-weight: bold;
+}
diff --git a/test/basic.expect.css b/test/basic.expect.css
new file mode 100644
index 0000000..f85069a
--- /dev/null
+++ b/test/basic.expect.css
@@ -0,0 +1,21 @@
+.modal {
+	border: thick dotted red;
+	color: red;
+}
+
+.modal:hover {
+	outline: none;
+}
+
+.serious-modal {
+	border: thick dotted red;
+	color: red;
+}
+
+.serious-modal:hover {
+	outline: none;
+}
+
+.serious-modal {
+	font-weight: bold;
+}
diff --git a/test/errors.css b/test/errors.css
new file mode 100644
index 0000000..5eb84e9
--- /dev/null
+++ b/test/errors.css
@@ -0,0 +1,19 @@
+test-does-not-extend-non-existent-selector {
+	@extend some-non-existent-selector;
+}
+
+test-does-not-extend-itself {
+	@extend test-does-not-extend-itself;
+}
+
+test-does-not-extend-itself-cleverly-1 {
+	@extend test-does-not-extend-itself-cleverly-2;
+}
+
+test-does-not-extend-itself-cleverly-2 {
+	@extend test-does-not-extend-itself-cleverly-1;
+}
+
+%test-placeholder {
+	@extend %test-placeholder;
+}
diff --git a/test/errors.expect.css b/test/errors.expect.css
new file mode 100644
index 0000000..86b6157
--- /dev/null
+++ b/test/errors.expect.css
@@ -0,0 +1,11 @@
+test-does-not-extend-non-existent-selector {
+}
+
+test-does-not-extend-itself {
+}
+
+test-does-not-extend-itself-cleverly-1 {
+}
+
+test-does-not-extend-itself-cleverly-2 {
+}
diff --git a/test/errors.ignore.expect.css b/test/errors.ignore.expect.css
new file mode 100644
index 0000000..c137b53
--- /dev/null
+++ b/test/errors.ignore.expect.css
@@ -0,0 +1,23 @@
+test-does-not-extend-non-existent-selector {
+	@extend some-non-existent-selector;
+}
+
+test-does-not-extend-itself {
+
+	@extend test-does-not-extend-itself
+}
+
+test-does-not-extend-itself-cleverly-1 {
+
+	@extend test-does-not-extend-itself-cleverly-1
+}
+
+test-does-not-extend-itself-cleverly-2 {
+
+	@extend test-does-not-extend-itself-cleverly-1
+}
+
+%test-placeholder {
+
+	@extend %test-placeholder
+}
diff --git a/test/errors.warn.expect.css b/test/errors.warn.expect.css
new file mode 100644
index 0000000..c137b53
--- /dev/null
+++ b/test/errors.warn.expect.css
@@ -0,0 +1,23 @@
+test-does-not-extend-non-existent-selector {
+	@extend some-non-existent-selector;
+}
+
+test-does-not-extend-itself {
+
+	@extend test-does-not-extend-itself
+}
+
+test-does-not-extend-itself-cleverly-1 {
+
+	@extend test-does-not-extend-itself-cleverly-1
+}
+
+test-does-not-extend-itself-cleverly-2 {
+
+	@extend test-does-not-extend-itself-cleverly-1
+}
+
+%test-placeholder {
+
+	@extend %test-placeholder
+}