diff --git a/src/compile/render-dom/Block.ts b/src/compile/render-dom/Block.ts
index bee2072ef1..947185d535 100644
--- a/src/compile/render-dom/Block.ts
+++ b/src/compile/render-dom/Block.ts
@@ -220,9 +220,10 @@ export default class Block {
 
 		if (this.hasIntroMethod || this.hasOutroMethod) {
 			this.addVariable('#current');
+			this.addVariable('#mounted');
 
 			if (!this.builders.mount.isEmpty()) {
-				this.builders.mount.addLine(`#current = true;`);
+				this.builders.mount.addLine(`#current = #mounted = true;`);
 			}
 
 			if (!this.builders.outro.isEmpty()) {
@@ -352,9 +353,8 @@ export default class Block {
 			} else {
 				properties.addBlock(deindent`
 					${dev ? 'i: function intro' : 'i'}(#target, anchor) {
-						if (#current) return;
 						${this.builders.intro}
-						this.m(#target, anchor);
+						if (!#mounted) this.m(#target, anchor);
 					},
 				`);
 			}
@@ -364,8 +364,6 @@ export default class Block {
 			} else {
 				properties.addBlock(deindent`
 					${dev ? 'o: function outro' : 'o'}(#outrocallback) {
-						if (!#current) return;
-
 						${this.outros > 1 && `#outrocallback = @callAfter(#outrocallback, ${this.outros});`}
 
 						${this.builders.outro}
diff --git a/src/compile/render-dom/wrappers/Element/index.ts b/src/compile/render-dom/wrappers/Element/index.ts
index cc479e55e7..b9bf2ef734 100644
--- a/src/compile/render-dom/wrappers/Element/index.ts
+++ b/src/compile/render-dom/wrappers/Element/index.ts
@@ -621,27 +621,24 @@ export default class ElementWrapper extends Wrapper {
 					? intro.expression.render(block)
 					: '{}';
 
-				const fn = component.qualify(intro.name); // TODO add built-in transitions?
+				const fn = component.qualify(intro.name);
 
 				if (outro) {
-					block.builders.intro.addBlock(deindent`
-						if (${introName}) ${introName}.abort(1);
-						if (${outroName}) ${outroName}.abort(1);
+					block.builders.intro.addConditional(`@intros.enabled`, deindent`
+						@add_render_callback(() => {
+							if (${introName}) ${introName}.end();
+							${introName} = @create_in_transition(${this.var}, ${fn}, ${snippet});
+						});
+					`);
+				} else {
+					block.builders.intro.addConditional(`@intros.enabled`, deindent`
+						if (!${introName}) {
+							@add_render_callback(() => {
+								${introName} = @create_in_transition(${this.var}, ${fn}, ${snippet});
+							});
+						}
 					`);
 				}
-
-				block.builders.intro.addConditional(`@intros.enabled`, deindent`
-					@add_render_callback(() => {
-						${introName} = @create_transition(${this.var}, ${fn}, ${snippet}, true);
-						${introName}.run(1, () => {
-							${introName} = null;
-						});
-					});
-				`);
-
-				block.builders.outro.addBlock(deindent`
-					if (${introName}) ${introName}.abort();
-				`);
 			}
 
 			if (outro) {
@@ -653,17 +650,16 @@ export default class ElementWrapper extends Wrapper {
 				const fn = component.qualify(outro.name);
 
 				block.builders.intro.addBlock(deindent`
-					if (${outroName}) ${outroName}.abort(1);
+					if (${outroName}) ${outroName}.end();
 				`);
 
 				// TODO hide elements that have outro'd (unless they belong to a still-outroing
 				// group) prior to their removal from the DOM
 				block.builders.outro.addBlock(deindent`
-					${outroName} = @create_transition(${this.var}, ${fn}, ${snippet}, false);
-					${outroName}.run(0, #outrocallback);
+					${outroName} = @create_out_transition(${this.var}, ${fn}, ${snippet}, #outrocallback);
 				`);
 
-				block.builders.destroy.addConditional('detach', `if (${outroName}) ${outroName}.abort();`);
+				block.builders.destroy.addConditional('detach', `if (${outroName}) ${outroName}.end();`);
 			}
 		}
 	}
diff --git a/src/compile/render-dom/wrappers/shared/Tag.ts b/src/compile/render-dom/wrappers/shared/Tag.ts
index 510c2cf756..5fe91b74cc 100644
--- a/src/compile/render-dom/wrappers/shared/Tag.ts
+++ b/src/compile/render-dom/wrappers/shared/Tag.ts
@@ -27,6 +27,7 @@ export default class Tag extends Wrapper {
 		if (this.node.shouldCache) block.addVariable(value, snippet);
 
 		if (dependencies.size) {
+			// TODO can we check `#current` at the top of the `update` method, instead of in individual updates?
 			const changedCheck = (
 				(block.hasOutros ? `!#current || ` : '') +
 				[...dependencies].map((dependency: string) => `changed.${dependency}`).join(' || ')
diff --git a/src/internal/animations.js b/src/internal/animations.js
index 97d0cd734a..cf02827dd8 100644
--- a/src/internal/animations.js
+++ b/src/internal/animations.js
@@ -28,7 +28,7 @@ export function animate(node, from, fn, params) {
 		if (css) {
 			if (delay) node.style.cssText = cssText;
 
-			name = create_rule({ a: 0, b: 1, d: 1, duration }, easing, css);
+			name = create_rule(0, 1, duration, easing, css);
 
 			node.style.animation = (node.style.animation || '')
 				.split(', ')
diff --git a/src/internal/style_manager.js b/src/internal/style_manager.js
index cb0a2dde57..0ec992025e 100644
--- a/src/internal/style_manager.js
+++ b/src/internal/style_manager.js
@@ -13,12 +13,12 @@ function hash(str) {
 	return hash >>> 0;
 }
 
-export function create_rule({ a, b, d, duration }, ease, fn) {
+export function create_rule(a, b, duration, ease, fn) {
 	const step = 16.666 / duration;
 	let keyframes = '{\n';
 
 	for (let p = 0; p <= 1; p += step) {
-		const t = a + d * ease(p);
+		const t = a + (b - a) * ease(p);
 		keyframes += p * 100 + `%{${fn(t, 1 - t)}}\n`;
 	}
 
@@ -41,6 +41,8 @@ export function create_rule({ a, b, d, duration }, ease, fn) {
 }
 
 export function delete_rule(node, name) {
+	console.log(`delete ${name} from ${node.textContent}`, { active });
+
 	node.style.animation = node.style.animation
 		.split(', ')
 		.filter(anim => anim.indexOf(name) < 0)
@@ -53,6 +55,7 @@ export function clear_rules() {
 	requestAnimationFrame(() => {
 		if (active) return;
 		let i = stylesheet.cssRules.length;
+		console.log(`clear_rules ${i}`);
 		while (i--) stylesheet.deleteRule(i);
 		current_rules = {};
 	});
diff --git a/src/internal/transitions.js b/src/internal/transitions.js
index 9b3e897960..f353bd5112 100644
--- a/src/internal/transitions.js
+++ b/src/internal/transitions.js
@@ -1,4 +1,4 @@
-import { identity as linear, noop, run } from './utils.js';
+import { identity as linear, noop, run, run_all } from './utils.js';
 import { loop } from './loop.js';
 import { create_rule, delete_rule } from './style_manager.js';
 
@@ -24,6 +24,127 @@ export function group_outros() {
 	};
 }
 
+export function create_in_transition(node, fn, params) {
+	let config = fn(node, params);
+	let running = true;
+	let animation_name;
+
+	function cleanup() {
+		delete_rule(node, animation_name);
+	}
+
+	wait().then(() => {
+		if (typeof config === 'function') config = config();
+
+		const {
+			delay = 0,
+			duration = 300,
+			easing = linear,
+			tick = noop,
+			css
+		} = config;
+
+		if (css) {
+			animation_name = create_rule(0, 1, duration, easing, css);
+			node.style.animation = (node.style.animation ? ', ' : '') + `${animation_name} ${duration}ms linear ${delay}ms 1 both`;
+		}
+
+		tick(0, 1);
+
+		const start_time = window.performance.now() + delay;
+		const end_time = start_time + duration;
+
+		loop(now => {
+			if (running) {
+				if (now > end_time) {
+					tick(1, 0);
+					cleanup();
+					return running = false;
+				}
+
+				if (now > start_time) {
+					const t = easing((now - start_time) / duration);
+					tick(t, 1 - t);
+				}
+			}
+
+			return running;
+		});
+	});
+
+	return {
+		end() {
+			if (running) {
+				cleanup();
+				running = false;
+			}
+		}
+	};
+}
+
+export function create_out_transition(node, fn, params, callback) {
+	let config = fn(node, params);
+	let running = true;
+	let animation_name;
+
+	const group = outros;
+
+	group.remaining += 1;
+	group.callbacks.push(callback); // TODO do we even need multiple callbacks? can we just have the one?
+
+	wait().then(() => {
+		if (typeof config === 'function') config = config();
+
+		const {
+			delay = 0,
+			duration = 300,
+			easing = linear,
+			tick = noop,
+			css
+		} = config;
+
+		if (css) {
+			animation_name = create_rule(1, 0, duration, easing, css);
+			node.style.animation += (node.style.animation ? ', ' : '') + `${animation_name} ${duration}ms linear ${delay}ms 1 both`;
+		}
+
+		const start_time = window.performance.now() + delay;
+		const end_time = start_time + duration;
+
+		loop(now => {
+			if (running) {
+				if (now > end_time) {
+					tick(0, 1);
+
+					if (!--group.remaining) {
+						// this will result in `end()` being called,
+						// so we don't need to clean up here
+						run_all(group.callbacks);
+					}
+
+					return false;
+				}
+
+				if (now > start_time) {
+					const t = easing((now - start_time) / duration);
+					tick(1 - t, t);
+				}
+			}
+
+			return running;
+		});
+	});
+
+	return {
+		end() {
+			if (running) {
+				delete_rule(node, animation_name);
+				running = false;
+			}
+		}
+	};
+}
+
 export function create_transition(node, fn, params, intro) {
 	let config = fn(node, params);
 
@@ -40,17 +161,17 @@ export function create_transition(node, fn, params, intro) {
 		animation_name = null;
 	}
 
-	function start(program, delay, duration, easing) {
+	function start(program, duration, easing) {
 		node.dispatchEvent(new window.CustomEvent(`${program.b ? 'intro' : 'outro'}start`));
 
 		program.a = t;
-		program.d = program.b - program.a;
-		program.duration = duration * Math.abs(program.b - program.a);
+		program.d = program.b - t;
+		program.duration = duration * Math.abs(program.d);
 		program.end = program.start + program.duration;
 
 		if (config.css) {
 			clear_animation();
-			animation_name = create_rule(program, easing, config.css);
+			animation_name = create_rule(t, program.b, program.duration, easing, config.css);
 
 			node.style.animation = (node.style.animation ? ', ' : '') + `${animation_name} ${program.duration}ms linear 1 forwards`;
 		}
@@ -101,7 +222,7 @@ export function create_transition(node, fn, params, intro) {
 		if (!ready) {
 			if (config.css && delay) {
 				// TODO can we just use the normal rule, but delay it?
-				animation_name = create_rule({ a: 0, b: 1, d: 1, duration: 1, }, linear, () => config.css(0, 1));
+				animation_name = create_rule(0, 1, 1, linear, () => config.css(0, 1));
 				node.style.animation = (node.style.animation ? ', ' : '') + `${animation_name} 9999s linear 1 both`;
 			}
 
@@ -117,7 +238,7 @@ export function create_transition(node, fn, params, intro) {
 		if (delay) {
 			pending_program = program;
 		} else {
-			start(program, delay, duration, easing);
+			start(program, duration, easing);
 		}
 
 		if (!running) {
@@ -129,7 +250,7 @@ export function create_transition(node, fn, params, intro) {
 				}
 
 				if (pending_program && now >= pending_program.start) {
-					start(pending_program, delay, duration, easing);
+					start(pending_program, duration, easing);
 				}
 
 				if (running) {