A browser app can keep its interface in one HTML file, use IndexedDB for structured data, and offer a small localStorage fallback—but those pieces have different limits. IndexedDB is asynchronous and suited to larger records; localStorage holds strings and is especially unreliable to assume for a document opened from a file: URL. Neither storage choice guarantees that data will survive quota limits, browser cleanup, or a device failure. A hand-written ZIP writer is a separate format-implementation task that needs its own validation.
Decide what “one offline HTML file” means
There are two different goals that are easy to confuse:
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- A self-contained interface: HTML, CSS, and JavaScript are packaged in one file, so the user can open the document without first downloading app assets from a server.
- Offline data: the app reads and writes information in browser storage without a network connection.
One does not guarantee the other. A hosted app may cache its interface assets with a service worker and the Cache API, while using IndexedDB for application data; MDN’s offline and background operation guide describes those as distinct roles. A standalone HTML file packages its interface differently, but its storage still depends on the browser and how the file is opened.
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| Property | IndexedDB | localStorage |
|---|---|---|
| Data shape | Structured records, including substantial client-side data and binary values. See MDN’s IndexedDB API guide. | String key-value pairs; objects must be serialized, typically as JSON. See MDN’s localStorage reference. |
| Capacity | Browser-managed quota and eviction policy; there is no single reliable quota figure for all browsers. See MDN’s storage quota guidance. | MDN documents a combined Web Storage maximum of 10 MiB per origin: up to 5 MiB for localStorage and 5 MiB for sessionStorage. This is a Web Storage limit, not an IndexedDB quota. |
| Operation model | Asynchronous; opening a database and running transactions can fail. | Synchronous API, intended for small amounts of string data. |
| Origin and direct-file behavior | Storage is scoped to the browser origin and is not a synchronization service. | Also origin-scoped, but behavior for file: URLs is undefined and may differ by browser. Do not rely on consistent localStorage behavior when users open the HTML file directly. |
Use IndexedDB as the main store when the app has structured records, larger data, or binary content. Reserve localStorage for small, serializable preferences or a limited fallback. A fallback is not a transparent replacement: the app must decide what data can be represented there, when to select it, and whether existing records need migration.
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Make fallback and failure behavior explicit
Storage may be unavailable or unusable: access can be denied, a quota can be reached, a database open can fail, or a transaction can abort or close. Browser-managed best-effort data can also be evicted, and users can clear site data. Treat these as normal states rather than assuming a successful write means permanent storage.
- Check whether the required storage mechanism is usable before presenting the app as ready to save.
- Handle database open and transaction errors, and tell the user when a save did not complete.
- If falling back to localStorage, restrict the fallback to data that fits its string-only model and supported size; explain any reduced capability rather than silently dropping records.
- Provide a user-controlled export and recovery path for information that matters. Offline persistence is not a backup.
For a directly opened file, test the exact browsers and launch path you intend to support. In particular, localStorage’s file: behavior is undefined, so a fallback that works for a hosted origin should not be assumed to work the same way from a local file.
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Keep offline asset delivery separate from offline storage
IndexedDB lets an app work with locally stored data while offline; it does not make a hosted app’s HTML, stylesheets, scripts, or other resources available after the network disappears. For a hosted origin that must start offline, cache the required assets separately, commonly with a service worker and the Cache API. A single-file app can carry its interface in the document itself, but that packaging choice does not remove storage, quota, or persistence limits.
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Treat the ZIP writer as its own format project
Browser compression support is not the same thing as ZIP support. The MDN Compression Streams API reference documents gzip and deflate streams; it does not establish that a custom writer produces valid ZIP archives or that ZIP readers will accept them.
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A ZIP implementation must be checked against the ZIP format requirements that apply to its intended files and target readers. Do not claim conformance or interoperability solely because the output downloads or because compression works. Validate the generated archive independently and test extraction with the readers and file types the app intends to support. The compression API documentation alone does not settle details such as archive records, checksums, filename encoding, or large-file extensions.
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