Switch the project to ES6 (via babel) & LESS (via lessc).

.babelrc

@@ -0,0 +1,7 @@
+{
+  "modules": "umd",
+  "moduleId": "tags-input",
+  "loose": "all",
+  "compact": true,
+  "comments": false
+}

.jshintrc

@@ -0,0 +1,4 @@
+{
+  "esnext": true,
+  "browser": true
+}

package.json

@@ -3,10 +3,26 @@
   "version": "0.8.0",
   "main": "tags-input.js",
   "description": "<input type=\"tags\"> like magic.",
+  "scripts": {
+    "build": "npm run transpile && npm run less && npm run size",
+    "transpile": "babel src -s -d .",
+    "less": "lessc --clean-css --source-map --autoprefix=\"last 2 versions\" src/tags-input.less tags-input.css",
+    "size": "echo \"gzip size: $(pretty-bytes $(gzip-size $npm_package_main))\"",
+    "test": "jshint src/**.js",
+    "prepublish": "npm run build",
+    "release": "npm run build && git commit -am $npm_package_version && git tag $npm_package_version && git push && git push --tags && npm publish"
+  },
   "repository": {
     "type": "git",
     "url": "git://github.com/developit/tags-input.git"
   },
-  "dependencies": {},
-  "devDependencies": {}
+  "devDependencies": {
+    "babel": "^5.8.21",
+    "gzip-size": "^3.0.0",
+    "jshint": "^2.8.0",
+    "less": "^2.5.1",
+    "less-plugin-autoprefix": "^1.4.2",
+    "less-plugin-clean-css": "^1.5.1",
+    "pretty-bytes": "^2.0.1"
+  }
 }

src/tags-input.js

@@ -0,0 +1,210 @@
+const BACKSPACE = 8,
+	TAB = 9,
+	ENTER = 13,
+	LEFT = 37,
+	RIGHT = 39,
+	DELETE = 46,
+	COMMA = 188;
+
+const SEPERATOR = ',';
+
+const COPY_PROPS = 'placeholder pattern spellcheck autocomplete autocapitalize autofocus accessKey accept lang minLength maxLength required'.split(' ');
+
+export default function tagsInput(input) {
+	function createElement(type, name, text, attributes) {
+		let el = document.createElement(type);
+		if (name) el.className = name;
+		if (text) el.textContent = text;
+		for (let key in attributes) {
+			el.setAttribute(`data-${key}`, attributes[key]);
+		}
+		return el;
+	}
+
+	function $(selector, all) {
+		return all ? Array.prototype.slice.call(base.querySelectorAll(selector)) : base.querySelector(selector);
+	}
+
+	function getValue() {
+		return $('.tag', true)
+			.map( tag => tag.textContent )
+			.concat(base.input.value || [])
+			.join(SEPERATOR);
+	}
+
+	function save() {
+		input.value = getValue();
+		input.dispatchEvent(new Event('change'));
+	}
+
+	// Return false if no need to add a tag
+	function addTag(text) {
+		// Add multiple tags if the user pastes in data with SEPERATOR already in it
+		if (~text.indexOf(SEPERATOR)) text = text.split(SEPERATOR);
+		if (Array.isArray(text)) return text.forEach(addTag);
+
+		let tag = text && text.trim();
+		// Ignore if text is empty
+		if (!tag) return false;
+
+		// For duplicates, briefly highlight the existing tag
+		if (!input.getAttribute('duplicates')) {
+			let exisingTag = $(`[data-tag="${tag}"]`);
+			if (exisingTag) {
+				exisingTag.classList.add('dupe');
+				setTimeout( () => exisingTag.classList.remove('dupe') , 100);
+				return false;
+			}
+		}
+
+		base.insertBefore(
+			createElement('span', 'tag', tag, { tag }),
+			base.input
+		);
+	}
+
+	function select(el) {
+		let sel = $('.selected');
+		if (sel) sel.classList.remove('selected');
+		if (el) el.classList.add('selected');
+	}
+
+	function setInputWidth() {
+		let last = $('.tag',true).pop(),
+			w = base.offsetWidth;
+		if (!w) return;
+		base.input.style.width = Math.max(
+			w - (last ? (last.offsetLeft+last.offsetWidth) : 5) - 5,
+			w/4
+		) + 'px';
+	}
+
+	function savePartialInput() {
+		if (addTag(base.input.value)!==false) {
+			base.input.value = '';
+			save();
+			setInputWidth();
+		}
+	}
+
+	function refocus(e) {
+		if (e.target.classList.contains('tag')) select(e.target);
+		if (e.target===base.input) return select();
+		base.input.focus();
+		e.preventDefault();
+		return false;
+	}
+
+	let base = createElement('div', 'tags-input'),
+		sib = input.nextSibling;
+	input.parentNode[sib?'insertBefore':'appendChild'](base, sib);
+
+	input.style.cssText = 'position:absolute;left:0;top:-99px;width:1px;height:1px;opacity:0.01;';
+	input.tabIndex = -1;
+
+	base.input = createElement('input');
+	base.input.setAttribute('type', 'text');
+	COPY_PROPS.forEach( prop => {
+		if (input.hasOwnProperty(prop)) {
+			base.input[prop] = input[prop];
+		}
+	});
+	base.appendChild(base.input);
+
+	delete input.pattern;
+	input.addEventListener('focus', () => {
+		base.input.focus();
+	});
+
+	base.input.addEventListener('focus', () => {
+		base.classList.add('focus');
+		select();
+	});
+
+	base.input.addEventListener('blur', () => {
+		base.classList.remove('focus');
+		select();
+		savePartialInput();
+	});
+
+	base.input.addEventListener('keydown', e => {
+		let key = e.keyCode || e.which,
+			selectedTag = $('.tag.selected'),
+			pos = this.selectionStart===this.selectionEnd && this.selectionStart,
+			last = $('.tag',true).pop();
+
+		setInputWidth();
+
+		if (key===ENTER || key===COMMA || key===TAB) {
+			if (!this.value && key!==COMMA) return;
+			savePartialInput();
+		}
+		else if (key===DELETE && selectedTag) {
+			if (selectedTag.nextSibling!==base.input) select(selectedTag.nextSibling);
+			base.removeChild(selectedTag);
+			setInputWidth();
+			save();
+		}
+		else if (key===BACKSPACE) {
+			if (selectedTag) {
+				select(selectedTag.previousSibling);
+				base.removeChild(selectedTag);
+				setInputWidth();
+				save();
+			}
+			else if (last && pos===0) {
+				select(last);
+			}
+			else {
+				return;
+			}
+		}
+		else if (key===LEFT) {
+			if (selectedTag) {
+				if (selectedTag.previousSibling) {
+					select(selectedTag.previousSibling);
+				}
+			}
+			else if (pos!==0) {
+				return;
+			}
+			else {
+				select(last);
+			}
+		}
+		else if (key===RIGHT) {
+			if (!selectedTag) {
+				return;
+			}
+			select(selectedTag.nextSibling);
+		}
+		else {
+			return select();
+		}
+
+		e.preventDefault();
+		return false;
+	});
+
+	// Proxy "input" (live change) events , update the first tag live as the user types
+	// This means that users who only want one thing don't have to enter commas
+	base.input.addEventListener('input', () => {
+		input.value = getValue();
+		input.dispatchEvent(new Event('input'));
+	});
+
+	// One tick after pasting, parse pasted text as CSV:
+	base.input.addEventListener('paste', () => {
+		setTimeout( () => savePartialInput(), 0);
+	});
+
+	base.addEventListener('mousedown', refocus);
+	base.addEventListener('touchstart', refocus);
+
+	// Add tags for existing values
+	input.value.split(SEPERATOR).forEach(addTag);
+	setInputWidth();
+}
+
+// make life easier:
+tagsInput.enhance = tagsInput.tagsInput = tagsInput;

src/tags-input.less

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+.tags-input {
+	display: inline-block;
+	padding: 0 2px;
+	background: #FFF;
+	border: 1px solid #CCC;
+	width: 16em;
+	border-radius: 2px;
+	box-shadow: inset 0 1px 2px rgba(0,0,0,0.1);
+
+	.tag {
+		display: inline-block;
+		background: #EEE;
+		color: #444;
+		padding: 0 4px;
+		margin: 2px;
+		border: 1px solid #CCC;
+		border-radius: 2px;
+		font: inherit;
+		user-select: none;
+		cursor: pointer;
+		transition: all 100ms ease;
+
+		&.selected {
+			background-color: #777;
+			border-color: #777;
+			color: #EEE;
+		}
+
+		&.dupe {
+			transform: scale3d(1.2,1.2,1.2);
+			background-color: #FCC;
+			border-color: #700;
+		}
+	}
+
+	input {
+		appearance: none !important;
+		display: inline-block !important;
+		padding: 3px;
+		margin: 0 !important;
+		background: none !important;
+		border: none !important;
+		box-shadow: none !important;
+		font: inherit !important;
+		font-size: 100% !important;
+		outline: none !important;
+	}
+
+	.selected ~ input {
+		opacity: 0.3;
+	}
+}