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PRESENTER STEPS:

  • Ensure your backup presenter is available
  • Present your slides or pre-recorded talk by:
    • selecting "Present now";
    • selecting "Your entire screen," "A window," or "Chrome tab;" and
    • selecting "Share"
  • If applicable, remove participants by:
    • [Non-Googlers] asking a Google participant to remove participants
    • [Googlers] selecting "Remove from meeting" in a participant's thumbnail image

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TECHNICAL CONSIDERATIONS:

  • Turn on captions by:
    • selecting "Turn on captions"
  • If you experience quality issues, change your send and receive resolutions to 360p by:
    • selecting "Settings"
    • selecting "Video" and then the setting you want to change

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Prerendering:

Design and Status

Matt Falkenhagen, on behalf of the team

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Prerender

Without

With

See also previous talk (slides) from BlinkOn 13

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Status and updates

  • Minimal core functionality mostly works ;)
    • Built on MPArch: see the BlinkOn 14 Talk just before this one
  • Origin Trial targeting Q3/Q4
    • Restricted to same-origin URL targets.
    • Triggered by the speculationrules API.

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Same-origin

  • Why
    • Reduces privacy/security risk.
    • Reduces compat risk: site opt-in.
  • Plan to support omnibox-triggered and cross-origin later.

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Speculation rules

  • New flexible, powerful API
  • Less risk of breakage than changing <link rel=prerender> behavior.

<script type="speculationrules">�{ prerender: [{ source: 'list', urls: ['next.html'] }] }�</script>

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Behaviors of prerendered content

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Restrictions on prerendered content

  • We restrict certain APIs used by prerendered content.
    • As part of the specification
  • Don’t want disruptive behavior: audio, popups, prompts, tab title change, etc.
  • Generally this applies to both same-origin and cross-origin
    • Some restrictions may vary

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Restricted APIs will mostly defer

document.addEventListener('prerenderingchange', () => {� console.log('activated!');�});

console.log(document.prerendering); // true;�const result = await navigator.frobinate();�console.log(document.prerendering); // false;

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Outputs: “true”, “activated!”, “false”.

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What APIs to restrict

  • APIs that already are restricted in some contexts are candidates for being restricted in prerendered content.
    • Permissions API-controlled features
    • Policy-controlled features (that are not permitted by default)

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Specification text

  • See in progress spec.
  • Example of deferral:

Modify the getCurrentPosition() method steps by prepending the following step:

If this's relevant global object's browsing context is a prerendering browsing context, then append the following steps to this's post-prerendering activation steps list and return.

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How this is implemented (idealized)

  • Blink-side

getCurrentPosition() {� if (document->IsPrerendering()) {

document->AddPostPrerenderActivationStep(

WTF::Bind(&getCurrentPosition));

return;�}

  • Browser-side
    • See lingqi@ and toyoshim@’s Day 3 lightning talk “Mojo capability control”

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WPT

  • Tests are landing at wpt_internal/prerender.
  • Before upstreaming we need a deterministic way to trigger prerendering in a test (speculationrules are just hints), and others.

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What have we done

  • We have identified 110 desired restrictions and finished 47% of them.
  • Many thanks to {myid.shin, gyuyoung, jkim} at Igalia for contributions.

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Special cases

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Storage

  • We plan to grant it during same-origin prerendering.
  • Sync storage APIs are very popular and cannot be deferred → grant.
  • Async APIs also granted for consistency.
  • Same-origin vs. Cross-origin
    • Cross-origin: privacy risk and lack of opt-in, needs a different solution.
  • Also considered isolated storage ideas + merging on click: may pursue in future (cross-origin may use this).

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Session storage

  • Problem: Session storage is intended to be scoped to the “tab”. Is the prerendered page in the tab?
  • Problem: If so, there are now multiple live pages in the same “tab”. Do all these pages use the same storage?
  • Problem: Current browsers cannot handle multiple processes using the same session storage.
  • Proposal:

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Service worker

  • Problem: Service workers can intercept resource requests. Should service worker see prerender navigations and loads?
  • Problem: Service workers can see open pages. Should service worker see prerendered pages?
  • Problem: Can a service worker started for prerendering be used to bypass the API restrictions?
  • Proposal:
    • SW can see prerender navigations.
    • SW can see prerendered pages as clients.
    • “Prerendered SW” is restricted. SW counts as “prerendered SW” iff all its clients are prerendered.

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Session history

  • Problem: What does history.back() do in a prerendered context?
    • Does it go to the previous state in the prerendered context?
  • Problem: What does history.back() do after activation occurred?
    • Does it go to the previous state from the prerendered context, or to the previous state from the primary context?
  • Problem: What does history.length look like before and after activation?
  • Proposal:
    • Prerendered page's window.history.length is always 1.
    • When a prerendered page is activated, its solo history entry is appended to the joint session history.

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Try it now

  • chrome://flags/#enable-prerender2 (or --enable-features=prerender2)
    • Uses <link rel=prerender> currently, and same-origin only.
  • Demo page: https://prerender2.glitch.me/ (from arthursonzogni@)
  • Bugs are tracked at Internals>Preload>Prerender
  • See design doc
  • We are looking for help! Contact falken@chromium.org.

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Discussion

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Archive

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Agenda

  • What is Prerendering
  • Launch plan
  • Same-origin
  • Speculation rules
  • Restrictions/behaviors of prerendered content
    • Session storage
    • Service worker
    • Session history
  • Specification
  • Testing
  • Metrics

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Metrics (maybe drop this one)

  • TODO add text

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