diff --git a/index.html b/index.html
index a400eff88..4257b3729 100644
--- a/index.html
+++ b/index.html
@@ -16,6 +16,9 @@
 	<strong>We're sorry but Vikunja doesn't work properly without JavaScript enabled. Please enable it to continue.</strong>
 </noscript>
 <div id="app"></div>
+<!-- TODO_OFFLINE maybe need to dynamically import it instead.
+Not sure if it works ok inside Delta Chat. -->
+<script type="module" src="/webxdc.js"></script>
 <script type="module" src="/src/main.ts"></script>
 <script>
     //
diff --git a/package.json b/package.json
index f558f656e..460bd64d0 100644
--- a/package.json
+++ b/package.json
@@ -91,6 +91,9 @@
     "vue-i18n": "9.2.2",
     "vue-router": "4.1.6",
     "workbox-precaching": "6.5.4",
+    "y-indexeddb": "^9.0.10",
+    "y-webrtc": "^10.2.5",
+    "yjs": "^13.5.51",
     "zhyswan-vuedraggable": "4.1.3"
   },
   "devDependencies": {
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index fbe3fb90f..a1f68485e 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -100,6 +100,9 @@ specifiers:
   wait-on: 7.0.1
   workbox-cli: 6.5.4
   workbox-precaching: 6.5.4
+  y-indexeddb: ^9.0.10
+  y-webrtc: ^10.2.5
+  yjs: ^13.5.51
   zhyswan-vuedraggable: 4.1.3
 
 dependencies:
@@ -147,6 +150,9 @@ dependencies:
   vue-i18n: 9.2.2_vue@3.2.47
   vue-router: 4.1.6_vue@3.2.47
   workbox-precaching: 6.5.4
+  y-indexeddb: 9.0.10_yjs@13.5.51
+  y-webrtc: 10.2.5
+  yjs: 13.5.51
   zhyswan-vuedraggable: 4.1.3_vue@3.2.47
 
 devDependencies:
@@ -5174,7 +5180,6 @@ packages:
 
   /base64-js/1.5.1:
     resolution: {integrity: sha512-AKpaYlHn8t4SVbOHCy+b5+KKgvR4vrsD8vbvrbiQJps7fKDTkjkDry6ji0rUJjC0kzbNePLwzxq8iypo41qeWA==}
-    dev: true
 
   /bcrypt-pbkdf/1.0.2:
     resolution: {integrity: sha512-qeFIXtP4MSoi6NLqO12WfqARWWuCKi2Rn/9hJLEmtB5yTNr9DqFWkJRCf2qShWzPeAMRnOgCrq0sg/KLv5ES9w==}
@@ -5417,7 +5422,6 @@ packages:
     dependencies:
       base64-js: 1.5.1
       ieee754: 1.2.1
-    dev: true
 
   /builtin-modules/3.2.0:
     resolution: {integrity: sha512-lGzLKcioL90C7wMczpkY0n/oART3MbBa8R9OFGE1rJxoVI86u4WAGfEk8Wjv10eKSyTHVGkSo3bvBylCEtk7LA==}
@@ -7188,6 +7192,10 @@ packages:
     hasBin: true
     dev: true
 
+  /err-code/3.0.1:
+    resolution: {integrity: sha512-GiaH0KJUewYok+eeY05IIgjtAe4Yltygk9Wqp1V5yVWLdhf0hYZchRjNIT9bb0mSwRcIusT3cx7PJUf3zEIfUA==}
+    dev: false
+
   /error-ex/1.3.2:
     resolution: {integrity: sha512-7dFHNmqeFSEt2ZBsCriorKnn3Z2pj+fd9kmI6QoWw4//DL+icEBfc0U7qJCisqrTsKTjw4fNFy2pW9OqStD84g==}
     dependencies:
@@ -8338,6 +8346,10 @@ packages:
       node-source-walk: 5.0.0
     dev: true
 
+  /get-browser-rtc/1.1.0:
+    resolution: {integrity: sha512-MghbMJ61EJrRsDe7w1Bvqt3ZsBuqhce5nrn/XAwgwOXhcsz53/ltdxOse1h/8eKXj5slzxdsz56g5rzOFSGwfQ==}
+    dev: false
+
   /get-caller-file/2.0.5:
     resolution: {integrity: sha512-DyFP3BM/3YHTQOCUL/w0OZHR0lpKeGrxotcHWcqNEdnltqFwXVfhEBQ94eIo34AfQpo0rGki4cyIiftY06h2Fg==}
     engines: {node: 6.* || 8.* || >= 10.*}
@@ -9067,7 +9079,6 @@ packages:
 
   /ieee754/1.2.1:
     resolution: {integrity: sha512-dcyqhDvX1C46lXZcVqCpK+FtMRQVdIMN6/Df5js2zouUsqG7I6sFxitIC+7KYK29KdXOLHdu9zL4sFnoVQnqaA==}
-    dev: true
 
   /ignore/4.0.6:
     resolution: {integrity: sha512-cyFDKrqc/YdcWFniJhzI42+AzS+gNwmUzOSFcRCQYwySuBBBy/KjuxWLZ/FHEH6Moq1NizMOBWyTcv8O4OZIMg==}
@@ -9130,7 +9141,6 @@ packages:
 
   /inherits/2.0.4:
     resolution: {integrity: sha512-k/vGaX4/Yla3WzyMCvTQOXYeIHvqOKtnqBduzTHpzpQZzAskKMhZ2K+EnBiSM9zGSoIFeMpXKxa4dYeZIQqewQ==}
-    dev: true
 
   /ini/1.3.7:
     resolution: {integrity: sha512-iKpRpXP+CrP2jyrxvg1kMUpXDyRUFDWurxbnVT1vQPx+Wz9uCYsMIqYuSBLV+PAaZG/d7kRLKRFc9oDMsH+mFQ==}
@@ -9676,6 +9686,10 @@ packages:
     engines: {node: '>=0.10.0'}
     dev: true
 
+  /isomorphic.js/0.2.5:
+    resolution: {integrity: sha512-PIeMbHqMt4DnUP3MA/Flc0HElYjMXArsw1qwJZcm9sqR8mq3l8NYizFMty0pWwE/tzIGH3EKK5+jes5mAr85yw==}
+    dev: false
+
   /isstream/0.1.2:
     resolution: {integrity: sha512-Yljz7ffyPbrLpLngrMtZ7NduUgVvi6wG9RJ9IUcyCd59YQ911PBJphODUcbOVbqYfxe1wuYf/LJ8PauMRwsM/g==}
     dev: true
@@ -10068,6 +10082,14 @@ packages:
       type-check: 0.4.0
     dev: true
 
+  /lib0/0.2.73:
+    resolution: {integrity: sha512-aJJIElCLWnHMcYZPtsM07QoSfHwpxCy4VUzBYGXFYEmh/h2QS5uZNbCCfL0CqnkOE30b7Tp9DVfjXag+3qzZjQ==}
+    engines: {node: '>=14'}
+    hasBin: true
+    dependencies:
+      isomorphic.js: 0.2.5
+    dev: false
+
   /light-my-request/5.8.0:
     resolution: {integrity: sha512-4BtD5C+VmyTpzlDPCZbsatZMJVgUIciSOwYhJDCbLffPZ35KoDkDj4zubLeHDEb35b4kkPeEv5imbh+RJxK/Pg==}
     dependencies:
@@ -12626,7 +12648,6 @@ packages:
     resolution: {integrity: sha512-vYl3iOX+4CKUWuxGi9Ukhie6fsqXqS9FE2Zaic4tNFD2N2QQaXOMFbuKK4QmDHC0JO6B1Zp41J0LpT0oR68amQ==}
     dependencies:
       safe-buffer: 5.2.1
-    dev: true
 
   /range-parser/1.2.1:
     resolution: {integrity: sha512-Hrgsx+orqoygnmhFbKaHE6c296J+HTAQXoxEF6gNupROmmGJRoyzfG3ccAveqCBrwr/2yxQ5BVd/GTl5agOwSg==}
@@ -12714,7 +12735,6 @@ packages:
       inherits: 2.0.4
       string_decoder: 1.3.0
       util-deprecate: 1.0.2
-    dev: true
 
   /readable-stream/4.3.0:
     resolution: {integrity: sha512-MuEnA0lbSi7JS8XM+WNJlWZkHAAdm7gETHdFK//Q/mChGyj2akEFtdLZh32jSdkWGbRwCW9pn6g3LWDdDeZnBQ==}
@@ -13097,7 +13117,6 @@ packages:
 
   /safe-buffer/5.2.1:
     resolution: {integrity: sha512-rp3So07KcdmmKbGvgaNxQSJr7bGVSVk5S9Eq1F+ppbRo70+YeaDxkw5Dd8NPN+GD6bjnYm2VuPuCXmpuYvmCXQ==}
-    dev: true
 
   /safe-json-stringify/1.2.0:
     resolution: {integrity: sha512-gH8eh2nZudPQO6TytOvbxnuhYBOvDBBLW52tz5q6X58lJcd/tkmqFR+5Z9adS8aJtURSXWThWy/xJtJwixErvg==}
@@ -13300,6 +13319,20 @@ packages:
     resolution: {integrity: sha512-wnD2ZE+l+SPC/uoS0vXeE9L1+0wuaMqKlfz9AMUo38JsyLSBWSFcHR1Rri62LZc12vLr1gb3jl7iwQhgwpAbGQ==}
     dev: true
 
+  /simple-peer/9.11.1:
+    resolution: {integrity: sha512-D1SaWpOW8afq1CZGWB8xTfrT3FekjQmPValrqncJMX7QFl8YwhrPTZvMCANLtgBwwdS+7zURyqxDDEmY558tTw==}
+    dependencies:
+      buffer: 6.0.3
+      debug: 4.3.4
+      err-code: 3.0.1
+      get-browser-rtc: 1.1.0
+      queue-microtask: 1.2.3
+      randombytes: 2.1.0
+      readable-stream: 3.6.0
+    transitivePeerDependencies:
+      - supports-color
+    dev: false
+
   /simple-swizzle/0.2.2:
     resolution: {integrity: sha512-JA//kQgZtbuY83m+xT+tXJkmJncGMTFT+C+g2h2R9uxkYIrE2yy9sgmcLhCnw57/WSD+Eh3J97FPEDFnbXnDUg==}
     dependencies:
@@ -13712,7 +13745,6 @@ packages:
     resolution: {integrity: sha512-hkRX8U1WjJFd8LsDJ2yQ/wWWxaopEsABU1XfkM8A+j0+85JAGppt16cr1Whg6KIbb4okU6Mql6BOj+uup/wKeA==}
     dependencies:
       safe-buffer: 5.2.1
-    dev: true
 
   /stringify-object/3.3.0:
     resolution: {integrity: sha512-rHqiFh1elqCQ9WPLIC8I0Q/g/wj5J1eMkyoiD6eoQApWHP0FtlK7rqnhmabL5VUY9JQCcqwwvlOaSuutekgyrw==}
@@ -14647,7 +14679,6 @@ packages:
 
   /util-deprecate/1.0.2:
     resolution: {integrity: sha512-EPD5q1uXyFxJpCrLnCc1nHnq3gOa6DZBocAIiI2TaSCA7VCJ1UJDMagCzIkXNsUYfD1daK//LTEQ8xiIbrHtcw==}
-    dev: true
 
   /utils-merge/1.0.1:
     resolution: {integrity: sha512-pMZTvIkT1d+TFGvDOqodOclx0QWkkgi6Tdoa8gC8ffGAAqz9pzPTZWAybbsHHoED/ztMtkv/VoYTYyShUn81hA==}
@@ -15416,6 +15447,21 @@ packages:
       signal-exit: 3.0.7
     dev: true
 
+  /ws/7.5.9:
+    resolution: {integrity: sha512-F+P9Jil7UiSKSkppIiD94dN07AwvFixvLIj1Og1Rl9GGMuNipJnV9JzjD6XuqmAeiswGvUmNLjr5cFuXwNS77Q==}
+    engines: {node: '>=8.3.0'}
+    requiresBuild: true
+    peerDependencies:
+      bufferutil: ^4.0.1
+      utf-8-validate: ^5.0.2
+    peerDependenciesMeta:
+      bufferutil:
+        optional: true
+      utf-8-validate:
+        optional: true
+    dev: false
+    optional: true
+
   /ws/8.11.0:
     resolution: {integrity: sha512-HPG3wQd9sNQoT9xHyNCXoDUa+Xw/VevmY9FoHyQ+g+rrMn4j6FB4np7Z0OhdTgjx6MgQLK7jwSy1YecU1+4Asg==}
     engines: {node: '>=10.0.0'}
@@ -15466,6 +15512,37 @@ packages:
     engines: {node: '>=0.4'}
     dev: true
 
+  /y-indexeddb/9.0.10_yjs@13.5.51:
+    resolution: {integrity: sha512-Tz4IzLZ20Pe8LTjXU125k2+7caERsX4ANsrjskHMm3yXUX4v9kBga/kK7ctT05P0uAj+glQComkAglY1qey7zg==}
+    peerDependencies:
+      yjs: ^13.0.0
+    dependencies:
+      lib0: 0.2.73
+      yjs: 13.5.51
+    dev: false
+
+  /y-protocols/1.0.5:
+    resolution: {integrity: sha512-Wil92b7cGk712lRHDqS4T90IczF6RkcvCwAD0A2OPg+adKmOe+nOiT/N2hvpQIWS3zfjmtL4CPaH5sIW1Hkm/A==}
+    dependencies:
+      lib0: 0.2.73
+    dev: false
+
+  /y-webrtc/10.2.5:
+    resolution: {integrity: sha512-ZyBNvTI5L28sQ2PQI0T/JvyWgvuTq05L21vGkIlcvNLNSJqAaLCBJRe3FHEqXoaogqWmRcEAKGfII4ErNXMnNw==}
+    engines: {node: '>=12'}
+    hasBin: true
+    dependencies:
+      lib0: 0.2.73
+      simple-peer: 9.11.1
+      y-protocols: 1.0.5
+    optionalDependencies:
+      ws: 7.5.9
+    transitivePeerDependencies:
+      - bufferutil
+      - supports-color
+      - utf-8-validate
+    dev: false
+
   /y18n/5.0.8:
     resolution: {integrity: sha512-0pfFzegeDWJHJIAmTLRP2DwHjdF5s7jo9tuztdQxAhINCdvS+3nGINqPd00AphqJR/0LhANUS6/+7SCb98YOfA==}
     engines: {node: '>=10'}
@@ -15529,6 +15606,13 @@ packages:
       fd-slicer: 1.1.0
     dev: true
 
+  /yjs/13.5.51:
+    resolution: {integrity: sha512-F1Nb3z3TdandD80IAeQqgqy/2n9AhDLcXoBhZvCUX1dNVe0ef7fIwi6MjSYaGAYF2Ev8VcLcsGnmuGGOl7AWbw==}
+    engines: {node: '>=16.0.0', npm: '>=8.0.0'}
+    dependencies:
+      lib0: 0.2.73
+    dev: false
+
   /yn/3.1.1:
     resolution: {integrity: sha512-Ux4ygGWsu2c7isFWe8Yu1YluJmqVhxqK2cLXNQA5AcC3QfbGNpM7fu0Y8b/z16pXLnFxZYvWhd3fhBY9DLmC6Q==}
     engines: {node: '>=6'}
diff --git a/src/localBackend/buckets.ts b/src/localBackend/buckets.ts
index 3fb54a8cc..aeb33a14a 100644
--- a/src/localBackend/buckets.ts
+++ b/src/localBackend/buckets.ts
@@ -1,4 +1,5 @@
 import type { IBucket } from '@/modelTypes/IBucket'
+import { ensureCreateSyncedYDoc } from './sync'
 import { getAllTasks, getTasksOfBucket } from './tasks'
 import { defaultPositionIfZero } from './utils/calculateDefaultPosition'
 
@@ -59,15 +60,20 @@ function getInitialBuckets(): IBucketWithoutTasks[] {
 	]
 }
 
-function getAllBucketsWithoutTasks(): IBucketWithoutTasks[] {
-	const fromStorage = localStorage.getItem('buckets')
-	if (!fromStorage) {
+async function getAllBucketsWithoutTasks(): Promise<IBucketWithoutTasks[]> {
+	const fromYdoc = (await ensureCreateSyncedYDoc())
+		// Yes, a map with an only element. I'm too lazy to make it a Y.Array.
+		.getMap('bucketsMap')
+		.get('buckets')
+
+	// const fromStorage = localStorage.getItem('buckets')
+	if (!fromYdoc) {
 		// TODO_OFFLINE dynamic import.
 		// Currently we have a constant list. Each project must have at least one bucket
 		return getInitialBuckets()
 	}
 	// TODO_OFFLINE fill the `tasks`.
-	return JSON.parse(fromStorage)
+	return fromYdoc
 }
 
 // /**
@@ -77,23 +83,23 @@ function getAllBucketsWithoutTasks(): IBucketWithoutTasks[] {
 //   (bucket as IBucket).tasks = getAllTas
 // }
 
-function getAllBuckets(): IBucket[] {
-	const bucketsWithoutTasks = getAllBucketsWithoutTasks()
-	const buckets = bucketsWithoutTasks.map(bucketWithoutTasks => {
+async function getAllBuckets(): Promise<IBucket[]> {
+	const bucketsWithoutTasks = await getAllBucketsWithoutTasks()
+	const buckets = bucketsWithoutTasks.map(async bucketWithoutTasks => {
 		const b = bucketWithoutTasks as IBucket
-		b.tasks = getTasksOfBucket(b.id)
+		b.tasks = (await getTasksOfBucket(b.id))
 			// Tasks are always sorted by their `kanbanPosition`.
 			// https://kolaente.dev/vikunja/api/src/commit/6d8db0ce1e00e8c200a43b28ac98eb0fb825f4d4/pkg/models/kanban.go#L173-L178
 			.sort((a, b) => a.kanbanPosition - b.kanbanPosition)
 		return b
 	})
-	return buckets
+	return Promise.all(buckets)
 }
 
-export function getAllBucketsOfProject(projectId: number): IBucket[] {
+export async function getAllBucketsOfProject(projectId: number): Promise<IBucket[]> {
 	// TODO_OFFLINE filter by position bruh.
 	// TODO_OFFLINE perf: maybe it's not worth getting tasks of each project then in `getAllBuckets`.
-	return getAllBuckets()
+	return (await getAllBuckets())
 		.filter(b => b.projectId === projectId)
 		// Buckets are always sorted.
 		// https://kolaente.dev/vikunja/api/src/commit/6d8db0ce1e00e8c200a43b28ac98eb0fb825f4d4/pkg/models/kanban.go#L139-L143
@@ -101,15 +107,17 @@ export function getAllBucketsOfProject(projectId: number): IBucket[] {
 }
 
 /** https://kolaente.dev/vikunja/api/src/commit/769db0dab2e50bc477dec6c7e18309effc80a1bd/pkg/models/kanban.go#L80-L87 */
-export function getDefaultBucket(projectId: number): IBucket {
-	return getAllBucketsOfProject(projectId).sort((a, b) => a.position - b.position)[0]
+export async function getDefaultBucket(projectId: number): Promise<IBucket> {
+	return (await getAllBucketsOfProject(projectId))
+		.sort((a, b) => a.position - b.position)
+		[0]
 }
 
 /**
  * https://kolaente.dev/vikunja/api/src/commit/066c26f83e1e066bdc9d80b4642db1df0d6a77eb/pkg/models/kanban.go#L252-L267
  */
-export function createBucket(bucket: IBucket): IBucket {
-	const allBuckets = getAllBucketsWithoutTasks()
+export async function createBucket(bucket: IBucket): Promise<IBucket> {
+	const allBuckets = await getAllBucketsWithoutTasks()
 	const maxPosition = allBuckets.reduce((currMax, b) => {
 		return b.position > currMax
 			? b.position
@@ -126,13 +134,16 @@ export function createBucket(bucket: IBucket): IBucket {
 		// It's not actually necessary FYI, it will just taske extra space in the storage.
 		tasks: undefined,
 	}
-	allBuckets.push(newBucketFullDataToStore)
-	localStorage.setItem('buckets', JSON.stringify(allBuckets))
+	allBuckets.push(newBucketFullDataToStore);
+	(await ensureCreateSyncedYDoc())
+		.getMap('bucketsMap')
+		.set('buckets', allBuckets)
+	// localStorage.setItem('buckets', JSON.stringify(allBuckets))
 	return newBucketFullData
 }
 
-export function updateBucket(newBucketData: IBucket) {
-	const allBuckets = getAllBucketsWithoutTasks()
+export async function updateBucket(newBucketData: IBucket) {
+	const allBuckets = await getAllBucketsWithoutTasks()
 	// TODO_OFFLINE looks like the real backend also filters by prjectId, but
 	// since in localBackend all bucket `id`s are unique even between projects,
 	// it's not necessary
@@ -142,13 +153,17 @@ export function updateBucket(newBucketData: IBucket) {
 		return
 	}
 	//  TODO_OFFLINE remove tasks.
-	allBuckets.splice(targetBucketInd, 1, newBucketData)
-	localStorage.setItem('buckets', JSON.stringify(allBuckets))
+	newBucketData.tasks = undefined
+	allBuckets.splice(targetBucketInd, 1, newBucketData);
+	(await ensureCreateSyncedYDoc())
+		.getMap('bucketsMap')
+		.set('buckets', allBuckets)
+	// localStorage.setItem('buckets', JSON.stringify(allBuckets))
 	return newBucketData
 }
 
-export function deleteBucket({ id }: { id: number }) {
-	const allBuckets = getAllBucketsWithoutTasks()
+export async function deleteBucket({ id }: { id: number }) {
+	const allBuckets = await getAllBucketsWithoutTasks()
 
 	if (allBuckets.length <= 1) {
 		// Prevent removing the last bucket.
@@ -162,20 +177,37 @@ export function deleteBucket({ id }: { id: number }) {
 		return
 	}
 	const projectId = allBuckets[targetBucketInd].projectId
-	allBuckets.splice(targetBucketInd, 1)
-	localStorage.setItem('buckets', JSON.stringify(allBuckets))
+	allBuckets.splice(targetBucketInd, 1);
+	// localStorage.setItem('buckets', JSON.stringify(allBuckets))
+
+	const ydoc = await ensureCreateSyncedYDoc()
+	ydoc
+		.getMap('bucketsMap')
+		.set('buckets', allBuckets)
 
 	// Move all the tasks from this bucket to the default one.
 	// https://kolaente.dev/vikunja/api/src/commit/6aadaaaffc1fff4a94e35e8fa3f6eab397cbc3ce/pkg/models/kanban.go#L349-L353
 	const deletedBuckedId = id
-	const defaultBucketId = getDefaultBucket(projectId).id
-	const allTasks = getAllTasks()
-	allTasks.forEach(t => {
-		if (t.bucketId === deletedBuckedId) {
-			t.bucketId = defaultBucketId
-		}
+	const defaultBucketId = (await getDefaultBucket(projectId)).id
+
+	// const allTasks = getAllTasks()
+	// allTasks.forEach(t => {
+	// 	if (t.bucketId === deletedBuckedId) {
+	// 		t.bucketId = defaultBucketId
+	// 	}
+	// })
+	// localStorage.setItem('tasks', JSON.stringify(allTasks))
+	const tasksYarr = ydoc.getArray('tasks')
+	ydoc.transact(() => {
+		tasksYarr.forEach((t, i) => {
+			if (t.bucketId === deletedBuckedId) {
+				// TODO_OFFLINE not sure if it's ok to mutate the array while `forEach` ing it.
+				t.bucketId = defaultBucketId
+				tasksYarr.delete(i)
+				tasksYarr.insert(i, [t])
+			}
+		})
 	})
-	localStorage.setItem('tasks', JSON.stringify(allTasks))
 
 	// TODO_OFFLINE idk what it's supposed to return
 	return true
diff --git a/src/localBackend/sync.ts b/src/localBackend/sync.ts
new file mode 100644
index 000000000..5f4e745d8
--- /dev/null
+++ b/src/localBackend/sync.ts
@@ -0,0 +1,122 @@
+import * as Y from 'yjs'
+import { IndexeddbPersistence as YjsIndexeddbPersistence } from 'y-indexeddb'
+import type { Webxdc as WebxdcGeneric } from './webxdc'
+
+type Webxdc = WebxdcGeneric<{
+	update: number[],
+	sender: typeof webxdc.selfAddr
+}>
+declare const webxdc: Webxdc;
+// declare global {
+// 	interface Window {
+// 		webxdc: Webxdc;
+// 	}
+// }
+
+/**
+ * @returns resolves when all the pending updates have been applied
+ */
+function initWebxdcSyncProvider(ydoc: Y.Doc): Promise<void> {
+	// TODO_OFFLINE I'm not sure at all if this is correct. Namely:
+	// I'm not sure if it is right to never apply updates that we've already applied
+	// in the previos sessions, as it looks like that we rely on `IndexeddbPersistence`
+	// to save all the applied updates. What happens if it for some reason doesn't manage
+	// to not save some update? In theore we'd have to download it from other peers then, right?
+	// But we don't do it here.
+	//
+	// Better check how existing providers work, like `y-websocket`. Looks like they use
+	// `y-protocols/sync`.
+
+	// const lastAppliedWebxdcUpdateSerialNum = localStorage.getItem('__lastAppliedWebxdcUpdate')
+	const lastAppliedWebxdcUpdateSerialNum = 0
+
+	const setListenerP = webxdc.setUpdateListener(
+		(update) => {
+			// if (update.payload.sender === webxdc.selfAddr) {
+			// 	return
+			// }
+			console.log('update', update.serial)
+			// TODO_OFFLINE optimize this. Batch updates? ydoc.transact?
+			Y.applyUpdate(ydoc, new Uint8Array(update.payload.update), 'webxdcUpdateHandler')
+			// localStorage.setItem('__lastAppliedWebxdcUpdate', update.serial.toString())
+		},
+		lastAppliedWebxdcUpdateSerialNum
+			? parseInt(lastAppliedWebxdcUpdateSerialNum)
+			: 0,
+	)
+
+	// TODO_OFFLINE throttle
+	// TODO not sure if this reacts to the changes 
+	ydoc.on('update', (update, origin) => {
+		if (origin === 'webxdcUpdateHandler') {
+			return
+		}
+		// TODO_OFFLINE conversion to an ordinary array. Not good for performance.
+		// Look up how other Yjs connectors do this. Or
+		// https://docs.yjs.dev/api/document-updates#example-base64-encoding
+		const serializableArray = [...(update as Uint8Array)]
+		webxdc.sendUpdate({
+				payload: {
+					update: serializableArray,
+					sender: webxdc.selfAddr
+				},
+			},
+			''
+		)
+	})
+
+	return setListenerP
+}
+
+async function maybeInitWebrtcSync(ydoc: Y.Doc) {
+	if (localStorage.getItem('sharingEnabled') !== 'true') {
+		return
+	}
+	const sharingRoomName = localStorage.getItem('sharingRoomName')
+	if (sharingRoomName == null) {
+		return
+	}
+	const sharingRoomPassword = localStorage.getItem('sharingRoomPassword')
+	if (sharingRoomPassword == null) {
+		return
+	}
+	const { WebrtcProvider } = await import('y-webrtc')
+	// To avoid collisions when people are lazy about coming up with a long enough name.
+	const fullRoomName = `vikunja-${sharingRoomName}`
+	const provider = new WebrtcProvider(fullRoomName, ydoc, { password: sharingRoomPassword })
+	console.log('WebRTC sharing enabled')
+}
+
+async function createSyncedYDoc(): Promise<Y.Doc> {
+	const ydoc = new Y.Doc()
+
+	const indexeddbProvider = new YjsIndexeddbPersistence('p2p-tasks-db', ydoc)
+	// Did not check if we actually need to await
+	await indexeddbProvider.whenSynced
+
+	if (window.webxdc) {
+		// TODO_OFFLINE dynamic import
+		await initWebxdcSyncProvider(ydoc)
+	} else {
+		await maybeInitWebrtcSync(ydoc)
+	}
+
+
+	// const lastAppliedWebxdcUpdate = localStorage.getItem('__lastAppliedWebxdcUpdate')
+	// // function onWebxdcUpdate()
+	// const onWebxdcUpdate: Parameters<Webxdc['setUpdateListener']>[0] = (update) => {
+	// 	// update.
+	// }
+
+	return ydoc
+}
+
+let _ydoc: Promise<Y.Doc> | undefined = undefined
+export const ensureCreateSyncedYDoc = () => {
+	if (!_ydoc) {
+		// TODO_OFFLINE fix: if you call it twice in a row before it resolves,
+		// `createSyncedYDoc` will be exected twice. Don't await.
+		_ydoc = createSyncedYDoc()
+	}
+	return _ydoc
+}
diff --git a/src/localBackend/tasks.ts b/src/localBackend/tasks.ts
index 355e15639..377841e6a 100644
--- a/src/localBackend/tasks.ts
+++ b/src/localBackend/tasks.ts
@@ -1,7 +1,14 @@
 import type { ITask } from '@/modelTypes/ITask'
+import * as Y from 'yjs'
 import { getDefaultBucket } from './buckets'
+import { ensureCreateSyncedYDoc } from './sync'
 import { defaultPositionIfZero } from './utils/calculateDefaultPosition'
 
+// createSyncedYDoc().then(d => {
+// 	window.ydoc = d
+// 	console.log(d)
+// })
+
 // TODO_OFFLINE return types? ITask is not it, because of snake case and Date format.
 
 // TODO_OFFLINE actually `project_id` is not always present on a task.
@@ -23,12 +30,16 @@ function getUniquieInt() {
 /**
  * Yes, we store the data in camelCase.
  */
-export function getAllTasks(): ITask[] {
-	const fromStorage = localStorage.getItem('tasks')
-	if (!fromStorage) {
-		return []
-	}
-	const tasks: ITask[] = JSON.parse(fromStorage)
+export async function getAllTasks(): Promise<ITask[]> {
+	// const fromStorage = localStorage.getItem('tasks')
+	// if (!fromStorage) {
+	// 	return []
+	// }
+	// const tasks: ITask[] = JSON.parse(fromStorage)
+
+	const fromYdoc = (await ensureCreateSyncedYDoc()).getArray('tasks')
+
+	const tasks = fromYdoc.toArray() as ITask[]
 	// TODO_OFFLINE don't just always sort them by position but look at
 	// `parameters.sort_by`.
 	return tasks.sort((a, b) => a.position - b.position)
@@ -37,26 +48,29 @@ export function getAllTasks(): ITask[] {
 // getAllTasksWithFilters(params)
 
 // TODO_OFFLINE we only have one project currently, actually.
-export function getTasksOfProject<PID extends number>(projectId: PID): Array<ITask & { projectId: PID }> {
-	const tasks: ITask[] = getAllTasks().filter(t => t.projectId === projectId)
+export async function getTasksOfProject<PID extends number>(projectId: PID): Promise<(ITask & { projectId: PID })[]> {
+	const tasks: ITask[] = (await getAllTasks()).filter(t => t.projectId === projectId)
 	return tasks
 }
 
 /**
  * Unsorted
  */
-export function getTasksOfBucket<BID extends number>(
+export async function getTasksOfBucket<BID extends number>(
 	bucketId: BID,
-): Array<ITask & { bucketId: BID }> {
-	const tasks: ITask[] = getAllTasks().filter(t => t.bucketId === bucketId)
+): Promise<Array<ITask & { bucketId: BID} >> {
+	const tasks: ITask[] = (await getAllTasks()).filter(t => t.bucketId === bucketId)
 	return tasks
 }
 
 /**
  * https://kolaente.dev/vikunja/api/src/commit/066c26f83e1e066bdc9d80b4642db1df0d6a77eb/pkg/models/tasks.go#L913-L990
  */
-export function createTask(newTask: ITask): ITask {
-	const allTasks = getAllTasks()
+export async function createTask(newTask: ITask): Promise<ITask> {
+
+
+
+	// const allTasks = getAllTasks()
 	const newTaskFullData: ITask = {
 		...newTask,
 		id: Math.round(Math.random() * 1000000000000),
@@ -66,41 +80,95 @@ export function createTask(newTask: ITask): ITask {
 		// https://kolaente.dev/vikunja/api/src/commit/769db0dab2e50bc477dec6c7e18309effc80a1bd/pkg/models/tasks.go#L939-L940
 		bucketId: newTask.bucketId > 0
 			? newTask.bucketId
-			: getDefaultBucket(newTask.projectId).id,
-	}
-	allTasks.unshift(newTaskFullData)
-	localStorage.setItem('tasks', JSON.stringify(allTasks))
+			: (await getDefaultBucket(newTask.projectId)).id,
+	};
+
+	(await ensureCreateSyncedYDoc())
+		.getArray('tasks')
+		.unshift([newTaskFullData])
+		// .unshift([new Y.Map(newTaskFullData)])
+
+	// (await allTasks).unshift(newTaskFullData)
+	// localStorage.setItem('tasks', JSON.stringify(allTasks))
 	return newTaskFullData
 }
 
-export function getTask(taskId: number) {
-	return getAllTasks().find(t => t.id === taskId)
+export async function getTask(taskId: number) {
+	return (await getAllTasks()).find(t => t.id === taskId)
 }
 
-export function updateTask(newTaskData: ITask) {
+export async function updateTask(newTaskData: ITask) {
 	// TODO_OFFLINE a lot of stuff is not implemented. For example, marking a task "done"
 	// when it is moved to the "done" bucket.
 	// https://kolaente.dev/vikunja/api/src/commit/6aadaaaffc1fff4a94e35e8fa3f6eab397cbc3ce/pkg/models/tasks.go#L1008
-	const allTasks = getAllTasks()
-	const targetTaskInd = allTasks.findIndex(t => t.id === newTaskData.id)
+
+
+	// let targetTask
+	// const asd = (await ensureCreateSyncedYDoc()).getArray('tasks').forEach(_t => {
+	// 	// const t = _t as Y.Map<ITask>
+	// 	const t = _t
+	// 	if (t.get('id') === newTaskData.id) {
+	// 		targetTask = t
+	// 	}
+	// })
+
+	const tasksArr = (await ensureCreateSyncedYDoc()).getArray('tasks')
+
+	let targetTaskInd = -1
+	tasksArr.forEach((_t, i) => {
+		// const t = _t as Y.Map<ITask>
+		const t = _t
+		if (t.id === newTaskData.id) {
+			targetTaskInd = i;
+		}
+	})
+
+	// const allTasks = await getAllTasks()
+	// const targetTaskInd = allTasks.findIndex(t => t.id === newTaskData.id)
+
 	if (targetTaskInd < 0) {
 		console.warn('Tried to update a task, but it does not exist')
 		return
 	}
-	allTasks.splice(targetTaskInd, 1, newTaskData)
-	localStorage.setItem('tasks', JSON.stringify(allTasks))
+
+	tasksArr.doc!.transact(() => {
+		// TODO_OFFLINE consider mutating the task instead.
+		tasksArr.delete(targetTaskInd)
+		tasksArr.insert(targetTaskInd, [newTaskData])
+	})
+
+	// allTasks.splice(targetTaskInd, 1, newTaskData)
+	// localStorage.setItem('tasks', JSON.stringify(allTasks))
 	return newTaskData
 }
 
-export function deleteTask({ id }: { id: number }) {
-	const allTasks = getAllTasks()
-	const targetTaskInd = allTasks.findIndex(t => t.id === id)
+export async function deleteTask({ id }: { id: number }) {
+	const tasksArr = (await ensureCreateSyncedYDoc()).getArray('tasks')
+
+	let targetTaskInd = -1
+	tasksArr.forEach((_t, i) => {
+		// const t = _t as Y.Map<ITask>
+		const t = _t
+		if (t.id === id) {
+			targetTaskInd = i;
+		}
+	})
+
 	if (targetTaskInd < 0) {
 		console.warn('Tried to delete a task, but it does not exist')
 		return
 	}
-	allTasks.splice(targetTaskInd, 1)
-	localStorage.setItem('tasks', JSON.stringify(allTasks))
+
+	// tasksArr.delete(targetTaskInd)
+
+	// const allTasks = await getAllTasks()
+	// const targetTaskInd = allTasks.findIndex(t => t.id === id)
+	// if (targetTaskInd < 0) {
+	// 	console.warn('Tried to delete a task, but it does not exist')
+	// 	return
+	// }
+	// allTasks.splice(targetTaskInd, 1)
+	// localStorage.setItem('tasks', JSON.stringify(allTasks))
 
 	// TODO_OFFLINE idk what it's supposed to return
 	return true
diff --git a/src/localBackend/webxdc.d.ts b/src/localBackend/webxdc.d.ts
new file mode 100644
index 000000000..3c205b514
--- /dev/null
+++ b/src/localBackend/webxdc.d.ts
@@ -0,0 +1,71 @@
+//@ts-check
+
+type SendingStatusUpdate<T> = {
+  /** the payload, deserialized json:
+   * any javascript primitive, array or object. */
+  payload: T;
+  /** optional, short, informational message that will be added to the chat,
+   * eg. "Alice voted" or "Bob scored 123 in MyGame";
+   * usually only one line of text is shown,
+   * use this option sparingly to not spam the chat. */
+  info?: string;
+  /** optional, if the Webxdc creates a document, you can set this to the name of the document;
+   * do not set if the Webxdc does not create a document */
+  document?: string;
+  /** optional, short text, shown beside the icon;
+   * it is recommended to use some aggregated value,
+   * eg. "8 votes", "Highscore: 123" */
+  summary?: string;
+};
+
+type ReceivedStatusUpdate<T> = {
+  /** the payload, deserialized json */
+  payload: T;
+  /** the serial number of this update. Serials are larger than 0 and newer serials have higher numbers */
+  serial: number;
+  /** the maximum serial currently known */
+  max_serial: number;
+  /** optional, short, informational message. */
+  info?: string;
+  /** optional, if the Webxdc creates a document, this is the name of the document;
+   * not set if the Webxdc does not create a document */
+  document?: string;
+  /** optional, short text, shown beside the webxdc's icon. */
+  summary?: string;
+};
+
+interface Webxdc<T> {
+  /** Returns the peer's own address.
+   *  This is esp. useful if you want to differ between different peers - just send the address along with the payload,
+   *  and, if needed, compare the payload addresses against selfAddr() later on. */
+  selfAddr: string;
+  /** Returns the peer's own name. This is name chosen by the user in their settings, if there is nothing set, that defaults to the peer's address. */
+  selfName: string;
+  /**
+   * set a listener for new status updates.
+   * The "serial" specifies the last serial that you know about (defaults to 0).
+   * Note that own status updates, that you send with {@link sendUpdate}, also trigger this method
+   * @returns promise that resolves when the listener has processed all the update messages known at the time when `setUpdateListener` was called.
+   * */
+  setUpdateListener(cb: (statusUpdate: ReceivedStatusUpdate<T>) => void, serial?: number): Promise<void>;
+  /**
+   * @deprecated See {@link setUpdateListener|`setUpdateListener()`}.
+   */
+  getAllUpdates(): Promise<ReceivedStatusUpdate<T>[]>;
+  /**
+   * Webxdc are usually shared in a chat and run independently on each peer. To get a shared status, the peers use sendUpdate() to send updates to each other.
+   * @param update status update to send
+   * @param description short, human-readable description what this update is about. this is shown eg. as a fallback text in an email program.
+   */
+  sendUpdate(update: SendingStatusUpdate<T>, description: string): void;
+}
+
+////////// ANCHOR: global
+declare global {
+  interface Window {
+    webxdc: Webxdc<any>;
+  }
+}
+////////// ANCHOR_END: global
+
+export { SendingStatusUpdate, ReceivedStatusUpdate, Webxdc };
diff --git a/src/views/user/settings/General.vue b/src/views/user/settings/General.vue
index 50146cae1..254370299 100644
--- a/src/views/user/settings/General.vue
+++ b/src/views/user/settings/General.vue
@@ -1,5 +1,39 @@
 <template>
 	<card :title="$t('user.settings.general.title')" class="general-settings" :loading="loading">
+		<template v-if="isWebxdc">
+			<div class="field">
+				<label class="checkbox">
+					<input type="checkbox" v-model="sharingEnabled"/>
+					Enable sharing
+				</label>
+				<!-- TODO_OFFLINE a way to have multiple rooms, some private. -->
+				<p>Everyone who knows the room name and the password will have read and write access to the data over network.</p>
+				<p>You need to reload the page after changing the sharing settings.</p>
+			</div>
+			<template v-if="sharingEnabled">
+				<div class="field">
+					<label class="is-flex is-align-items-center">
+						Sharing: room name
+						<input
+							class="input"
+							type="text"
+							required
+							v-model="sharingRoomName"
+						/>
+					</label>
+				</div>
+				<div class="field">
+					<label class="is-flex is-align-items-center">
+						Sharing: room password
+						<input
+							class="input"
+							type="password"
+							v-model="sharingRoomPassword"
+						/>
+					</label>
+				</div>
+			</template>
+		</template>
 		<div class="field">
 			<label class="checkbox">
 				<input type="checkbox" v-model="playSoundWhenDone"/>
@@ -139,6 +173,11 @@ function getPlaySoundWhenDoneSetting() {
 }
 
 const playSoundWhenDone = ref(getPlaySoundWhenDoneSetting())
+// TODO_OFFLINE refactor: DRY
+const isWebxdc = window.webxdc == undefined
+const sharingEnabled = ref(localStorage.getItem('sharingEnabled') === 'true')
+const sharingRoomName = ref(localStorage.getItem('sharingRoomName') ?? '')
+const sharingRoomPassword = ref(localStorage.getItem('sharingRoomPassword') ?? '')
 const quickAddMagicMode = ref(getQuickAddMagicMode())
 
 const authStore = useAuthStore()
@@ -178,6 +217,13 @@ watch(
 
 async function updateSettings() {
 	localStorage.setItem(playSoundWhenDoneKey, playSoundWhenDone.value ? 'true' : 'false')
+	localStorage.setItem('sharingEnabled', sharingEnabled.value ? 'true' : 'false')
+	if (sharingRoomName.value.length > 0) {
+		localStorage.setItem('sharingRoomName', sharingRoomName.value)
+	}
+	if (sharingRoomPassword.value.length > 0) {
+		localStorage.setItem('sharingRoomPassword', sharingRoomPassword.value)
+	}
 	setQuickAddMagicMode(quickAddMagicMode.value)
 
 	await authStore.saveUserSettings({
diff --git a/vite.config.ts b/vite.config.ts
index 818b51519..06f0dd185 100644
--- a/vite.config.ts
+++ b/vite.config.ts
@@ -187,6 +187,9 @@ export default defineConfig(({mode}) => {
 		build: {
 			target: 'esnext',
 			rollupOptions: {
+				external: [
+					"/webxdc.js"
+				],
 				plugins: [
 					visualizer({
 						filename: 'stats.html',