Initial OPFS docs.

Author: Chriztiaan

client-sdk-references/javascript-web.mdx

@@ -307,6 +307,78 @@ powerSync.connect(connector)
 powerSync.connect(connector, { connectionMethod: SyncStreamConnectionMethod.HTTP });
 ```
 
+### Configuring Different SQLite Virtual Filesystems for Web
+The default SQLite VFS (`IDBBatchAtomicVFS`) can be replaced by supported [OPFS](https://developer.mozilla.org/en-US/docs/Web/API/File_System_API/Origin_private_file_system) alternatives.
+`IDBBatchAtomicVFS` uses `IndexedDB` as the underlying storage which may have worse performance and stability issues with browsers such as Safari when compared with `OPFS` based VFSs.
+See [WA-SQLite README](https://github.com/powersync-ja/wa-sqlite/blob/1bb58d3619b96a2708e0320e1c22d0f2b9db35c6/src/examples/README.md#L28) for more details on each option.
+
+`OPFS` can be configured with a `WASQLiteOpenFactory`.
+
+<CodeGroup>
+```js AccessHandlePoolVFS
+import { PowerSyncDatabase, WASQLiteOpenFactory, WASQLiteVFS } from '@powersync/web';
+
+export const db = new PowerSyncDatabase({
+  schema: AppSchema,
+  database: new WASQLiteOpenFactory({
+    dbFilename: 'exampleVFS.db',
+    vfs: WASQLiteVFS.AccessHandlePoolVFS,
+    flags: {
+      enableMultiTabs: typeof SharedWorker !== 'undefined'
+    } 
+  }),
+  flags: {
+    enableMultiTabs: typeof SharedWorker !== 'undefined'
+  }
+});
+```
+
+```js OPFSCoopSyncVFS (Safari Multiple Tabs Support)
+import { PowerSyncDatabase, WASQLiteOpenFactory, WASQLiteVFS } from '@powersync/web';
+
+export const db = new PowerSyncDatabase({
+  schema: AppSchema,
+  database: new WASQLiteOpenFactory({
+    dbFilename: 'exampleVFS.db',
+    vfs: WASQLiteVFS.OPFSCoopSyncVFS,
+    flags: {
+      enableMultiTabs: typeof SharedWorker !== 'undefined'
+    } 
+  }),
+  flags: {
+    enableMultiTabs: typeof SharedWorker !== 'undefined'
+  }
+});
+```
+</CodeGroup>
+
+**Note**: The `AccessHandlePoolVFS` does not to work correctly in the multiple tabs use case. `OPFSCoopSyncVFS` works with multiple tabs, and adds multiple tab support for both Safari and Safari iOS.
+| VFS Type               | Multi-Tab Support on Normal Browsers | Multi-Tab Support on Safari/Safari iOS | Notes                                                             |
+|-------------------------|-------------------------------------|----------------------------------------|-------------------------------------------------------------------|
+| **IDBBatchAtomicVFS**  | Supported                        | Not supported                        | May have stability issues on Safari. |
+| **AccessHandlePoolVFS**| Not supported                    | Not supported                       | Does not work correctly in multi-tab scenarios.                  |
+| **OPFSCoopSyncVFS**    | Supported                        | Supported                           | Works with multiple tabs on all browsers, including Safari.      |
+
+### HTTP Requirements
+
+The use of `OPFS` requires a secure context and additional HTTP headers when serving a web application.
+
+For Vite these headers can be configured in the Vite config file:
+`vite.config.mts`
+```js
+import { defineConfig } from 'vite';
+
+export default defineConfig({
+  server: {
+    headers: {
+      'Cross-Origin-Opener-Policy': 'same-origin',
+      'Cross-Origin-Embedder-Policy': 'require-corp',
+    },
+  },
+});
+
+```
+
 ## ORM Support
 
 See [JavaScript ORM Support](/client-sdk-references/javascript-web/javascript-orm/overview) for details.