diff --git a/.editorconfig b/.editorconfig
index ed2a319d58..90846de5ee 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -13,4 +13,3 @@ insert_final_newline = false
 
 [{package.json,.travis.yml,.eslintrc.json}]
 indent_style = space
-indent_size = 2
diff --git a/CHANGELOG.md b/CHANGELOG.md
index af1a97d1f9..5f9e04e32a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 # Svelte changelog
 
+## Unreleased
+
+* Custom Elements - Call `onMount` when connected & clean-up when disconnected ([#4522](https://github.com/sveltejs/svelte/pull/4522))
+
 ## 3.32.3
 
 * Fix removal of lone `:host` selectors ([#5982](https://github.com/sveltejs/svelte/issues/5982))
diff --git a/check_publish_env.js b/check_publish_env.js
new file mode 100644
index 0000000000..ce6f066cff
--- /dev/null
+++ b/check_publish_env.js
@@ -0,0 +1,4 @@
+if (!process.env.PUBLISH) {
+	console.error('npm publish must be run with the PUBLISH environment variable set');
+	process.exit(1);
+}
diff --git a/package.json b/package.json
index 333838e69d..5b04c3e82e 100644
--- a/package.json
+++ b/package.json
@@ -73,7 +73,7 @@
     "dev": "rollup -cw",
     "pretest": "npm run build",
     "posttest": "agadoo internal/index.mjs",
-    "prepublishOnly": "npm run lint && PUBLISH=true npm test",
+    "prepublishOnly": "node check_publish_env.js && npm run lint && npm test",
     "tsd": "tsc -p src/compiler --emitDeclarationOnly && tsc -p src/runtime --emitDeclarationOnly",
     "lint": "eslint \"{src,test}/**/*.{ts,js}\""
   },
diff --git a/src/compiler/compile/render_dom/index.ts b/src/compiler/compile/render_dom/index.ts
index 8e7deb7a78..a56f3fb2b3 100644
--- a/src/compiler/compile/render_dom/index.ts
+++ b/src/compiler/compile/render_dom/index.ts
@@ -485,7 +485,7 @@ export default function dom(
 
 					${css.code && b`this.shadowRoot.innerHTML = \`<style>${css.code.replace(/\\/g, '\\\\')}${options.dev ? `\n/*# sourceMappingURL=${css.map.toUrl()} */` : ''}</style>\`;`}
 
-					@init(this, { target: this.shadowRoot, props: ${init_props} }, ${definition}, ${has_create_fragment ? 'create_fragment' : 'null'}, ${not_equal}, ${prop_indexes}, ${dirty});
+					@init(this, { target: this.shadowRoot, props: ${init_props}, customElement: true }, ${definition}, ${has_create_fragment ? 'create_fragment' : 'null'}, ${not_equal}, ${prop_indexes}, ${dirty});
 
 					${dev_props_check}
 
diff --git a/src/runtime/internal/Component.ts b/src/runtime/internal/Component.ts
index 6d211dfedd..5a2a966d0b 100644
--- a/src/runtime/internal/Component.ts
+++ b/src/runtime/internal/Component.ts
@@ -34,6 +34,7 @@ interface T$$ {
 	on_mount: any[];
 	on_destroy: any[];
 	skip_bound: boolean;
+	on_disconnect: any[];
 }
 
 export function bind(component, name, callback) {
@@ -52,23 +53,26 @@ export function claim_component(block, parent_nodes) {
 	block && block.l(parent_nodes);
 }
 
-export function mount_component(component, target, anchor) {
+export function mount_component(component, target, anchor, customElement) {
 	const { fragment, on_mount, on_destroy, after_update } = component.$$;
 
 	fragment && fragment.m(target, anchor);
 
-	// onMount happens before the initial afterUpdate
-	add_render_callback(() => {
-		const new_on_destroy = on_mount.map(run).filter(is_function);
-		if (on_destroy) {
-			on_destroy.push(...new_on_destroy);
-		} else {
-			// Edge case - component was destroyed immediately,
-			// most likely as a result of a binding initialising
-			run_all(new_on_destroy);
-		}
-		component.$$.on_mount = [];
-	});
+	if (!customElement) {
+		// onMount happens before the initial afterUpdate
+		add_render_callback(() => {
+
+			const new_on_destroy = on_mount.map(run).filter(is_function);
+			if (on_destroy) {
+				on_destroy.push(...new_on_destroy);
+			} else {
+				// Edge case - component was destroyed immediately,
+				// most likely as a result of a binding initialising
+				run_all(new_on_destroy);
+			}
+			component.$$.on_mount = [];
+		});
+	}
 
 	after_update.forEach(add_render_callback);
 }
@@ -113,6 +117,7 @@ export function init(component, options, instance, create_fragment, not_equal, p
 		// lifecycle
 		on_mount: [],
 		on_destroy: [],
+		on_disconnect: [],
 		before_update: [],
 		after_update: [],
 		context: new Map(parent_component ? parent_component.$$.context : []),
@@ -155,7 +160,7 @@ export function init(component, options, instance, create_fragment, not_equal, p
 		}
 
 		if (options.intro) transition_in(component.$$.fragment);
-		mount_component(component, options.target, options.anchor);
+		mount_component(component, options.target, options.anchor, options.customElement);
 		flush();
 	}
 
@@ -173,6 +178,9 @@ if (typeof HTMLElement === 'function') {
 		}
 
 		connectedCallback() {
+			const { on_mount } = this.$$;
+			this.$$.on_disconnect = on_mount.map(run).filter(is_function);
+
 			// @ts-ignore todo: improve typings
 			for (const key in this.$$.slotted) {
 				// @ts-ignore todo: improve typings
@@ -184,6 +192,10 @@ if (typeof HTMLElement === 'function') {
 			this[attr] = newValue;
 		}
 
+		disconnectedCallback() {
+			run_all(this.$$.on_disconnect);
+		}
+
 		$destroy() {
 			destroy_component(this, 1);
 			this.$destroy = noop;
diff --git a/test/custom-elements/index.ts b/test/custom-elements/index.ts
index d3644a3142..35df156879 100644
--- a/test/custom-elements/index.ts
+++ b/test/custom-elements/index.ts
@@ -110,8 +110,8 @@ describe('custom-elements', function() {
 
 			const page = await browser.newPage();
 
-			page.on('console', (type, ...args) => {
-				console[type](...args);
+			page.on('console', (type) => {
+				console[type._type](type._text);
 			});
 
 			page.on('error', error => {
diff --git a/test/custom-elements/samples/oncreate/main.svelte b/test/custom-elements/samples/oncreate/main.svelte
index ed3980a28e..23819e660f 100644
--- a/test/custom-elements/samples/oncreate/main.svelte
+++ b/test/custom-elements/samples/oncreate/main.svelte
@@ -3,9 +3,12 @@
 <script>
 	import { onMount } from 'svelte';
 
-	export let wasCreated;
+  export let prop = false;
+  export let propsInitialized;
+  export let wasCreated;
 
-	onMount(() => {
-		wasCreated = true;
-	});
+  onMount(() => {
+    propsInitialized = prop !== false;
+    wasCreated = true;
+  });
 </script>
diff --git a/test/custom-elements/samples/oncreate/test.js b/test/custom-elements/samples/oncreate/test.js
index c33f8a6a10..f451979976 100644
--- a/test/custom-elements/samples/oncreate/test.js
+++ b/test/custom-elements/samples/oncreate/test.js
@@ -2,7 +2,9 @@ import * as assert from 'assert';
 import './main.svelte';
 
 export default function (target) {
-	target.innerHTML = '<my-app/>';
+	target.innerHTML = '<my-app prop/>';
 	const el = target.querySelector('my-app');
+
 	assert.ok(el.wasCreated);
+	assert.ok(el.propsInitialized);
 }
diff --git a/test/custom-elements/samples/ondestroy/main.svelte b/test/custom-elements/samples/ondestroy/main.svelte
new file mode 100644
index 0000000000..aa945ca602
--- /dev/null
+++ b/test/custom-elements/samples/ondestroy/main.svelte
@@ -0,0 +1,22 @@
+<svelte:options tag="my-app"/>
+
+<script>
+	import { onMount, onDestroy } from 'svelte';
+	
+	let el;
+	let parentEl;
+
+	onMount(() => {
+		parentEl = el.parentNode.host.parentElement;
+
+		return () => {
+			parentEl.dataset.onMountDestroyed = true;
+		}
+	});
+
+	onDestroy(() => {
+		parentEl.dataset.destroyed = true;
+	})
+</script>
+
+<div bind:this={el}></div>
diff --git a/test/custom-elements/samples/ondestroy/test.js b/test/custom-elements/samples/ondestroy/test.js
new file mode 100644
index 0000000000..61375bfa96
--- /dev/null
+++ b/test/custom-elements/samples/ondestroy/test.js
@@ -0,0 +1,11 @@
+import * as assert from 'assert';
+import './main.svelte';
+
+export default function (target) {
+	target.innerHTML = '<my-app/>';
+	const el = target.querySelector('my-app');
+	target.removeChild(el);
+
+	assert.ok(target.dataset.onMountDestroyed);
+	assert.equal(target.dataset.destroyed, undefined);
+}
diff --git a/test/js/samples/css-shadow-dom-keyframes/expected.js b/test/js/samples/css-shadow-dom-keyframes/expected.js
index 82a39e5924..4d188201eb 100644
--- a/test/js/samples/css-shadow-dom-keyframes/expected.js
+++ b/test/js/samples/css-shadow-dom-keyframes/expected.js
@@ -40,7 +40,8 @@ class Component extends SvelteElement {
 			this,
 			{
 				target: this.shadowRoot,
-				props: attribute_to_object(this.attributes)
+				props: attribute_to_object(this.attributes),
+				customElement: true
 			},
 			null,
 			create_fragment,