[Meta] A long term plan for subresource loading

Hi folks!

Let us share the current status and a future plan about Subresource Loading with Web Bundles.

Last update: Apr 2022

Status

• We're doing an origin trial in Chrome (M90 - M101). Landing page.

If we don't find any significant issue in OT and this feature is promising, as the next step, we'll work on addressing any
blocking issues or support feature requests, considering feedback from OT, so that we can ship
MVP (v1).

Tentative plan (No ETA):

Note: The following mentions only features. Any standard works or other tasks are not included.

v1

MVP, which proves that bundling multiple resources improves the loading performance than loading multiple resources separately using web native bundling format.

We consider MVP should be shipped with these features.

• The core part: Subresource Loading with Web Bundles
• The extension: Support opaque origin iframes
  • Change urn:uuid to other scheme (#650)
    • Related issue (#623)
• Make primary URL optional (#616)
  • Needs to update the bundle format.
• Support a relative URL of a resource in a bundle (#647)
  • Needs to update the bundle format? -> Clarified a relative URL is supported
• New syntax: Use <script> tag and define its scheme (#670)
• CORP and COEP (#644)
• ServiceWorker integration (#676)
  • WONT in v1
• Increment the bundle format version?
  • WONT. Note: Chromium uses "b2" internally until web bundle format goes to RFC.

v2

To cover more wider use cases, especially improving cache efficiency of each resources.
There are several proposals in this area and we'll explore this area in v2.

• Bundle Dependencies
• Index Section as Resources
• Subsetting (Bundle Preloading; formely known as Resource Batch Preloading, Resource Bundles)
  • Chromium side design doc, based on Resource Bundles proposal.

v3 (Strawperson ideas, which are not triaged well yet)

• Resource Hints with webbundle
  • <link rel=prefetch-for-navigation href=”//example.com/nextpage.wbn” page=”//example.com/nextpage.html”>
  • Needs a concrete proposal.
• Content Addressable Bundles

We'll try to keep this plan up-to-date.

Start by reading explainers/subresource-loading.md and the linked opaque-origin iframe explainer, then review the listed v2 proposals such as extensions/proposals/dependencies-section.md. Done would require a concrete, current long-term plan with its status and proposals updated, but this issue does not define a specific documentation change.