Worklets Level 1

Editor’s Draft,

This version:
https://drafts.css-houdini.org/worklets/
Previous Versions:
http://www.w3.org/TR/2016/WD-worklets-1-20160607/
Feedback:
public-houdini@w3.org with subject line “[worklets] … message topic …” (archives)
Issue Tracking:
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Abstract

This specification defines an API for running scripts in stages of the rendering pipeline independent of the main javascript execution environment.

Status of this document

This is a public copy of the editors’ draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

GitHub Issues are preferred for discussion of this specification. When filing an issue, please put the text “worklets” in the title, preferably like this: “[worklets] …summary of comment…”. All issues and comments are archived.

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 September 2015 W3C Process Document.

1. Introduction

1.1. Motivations

This section is not normative.

Allowing extension points defined in the document environment is difficult, as rendering engines would need to abandon previously held assumptions for what could happen in the middle of a phase.

For example, during the layout phase the rendering engine assumes that no DOM will be modified.

Additionally defining extension points in the document environment would restrict rendering engines to performing work in the same thread as the document environment. (Unless rendering engines added complex, high-overhead infrastructure to allow thread-safe APIs in addition to thread joining guarantees).

The worklet is designed to allow such extension points in rendering engines, while keeping guarantees which rendering engines rely currently on.

Worklets are similar to web workers however they:

As worklets have a relatively high overhead, they should be used sparingly. Due to this worklets are expected to be shared between separate scripts. This is similar to the document environment.

1.2. Code Idempotency

This section is not normative.

Multiple instances of WorkletGlobalScope can be created for each Worklet that they belong to. User agents may choose to do this in order to parallelize work over multiple threads, or to move work between threads as required.

Additionally different user agents may invoke a method on a class in a different order to other user agents.

As a result of this, to prevent this compatibility risk between user agents, authors who register classes on the global scope should make their code idempotent. That is, a method or set of methods on a class should produce the same output given a particular input.

The following techniques should be used in order to encourage authors to write code in an idempotent way:

2. Infrastructure

2.1. The Global Scope

The WorkletGlobalScope object provides a worklet global scope which represents the global execution context of a Worklet.

[Exposed=Worklet]
interface WorkletGlobalScope {
    attribute Console console;
};

A WorkletGlobalScope has an associated environment settings object.

Note: The WorkletGlobalScope has a limited global scope when compared to a DedicatedWorkerGlobalScope. It is expected that other specifications will extend WorkletGlobalScope with registerAClass methods which will allow authors to register classes for the user agent create and invoke methods on.

2.1.1. The event loop

Each WorkletGlobalScope object has a distinct event loop. This event loop has no associated browsing context, and only its microtask queue is used (all other task queues are not used). The event loop is created by the create a WorkletGlobalScope algorithm.

2.1.2. Creating a WorkletGlobalScope

When a user agent is to create a WorkletGlobalScope, for a given worklet, it must run the following steps:

  1. Let workletGlobalScopeType be the worklet’s worklet global scope type.

  2. Call the JavaScript InitializeHostDefinedRealm abstract operation with the following customizations:

    • For the global object, create a new workletGlobalScopeType object. Let workletGlobalScope be the created object.

    • Let realmExecutionContext be the created JavaScript execution context.

    • Do not obtain any source texts for scripts or modules.

  3. Let settingsObject be the result of set up a worklet environment settings object with realmExecutionContext.

  4. Associate the settingsObject with workletGlobalScope.

  5. For each entry in the given worklet’s module responses map (in insertion order), run the following substeps:

    1. Let resolvedModuleURL be entry’s key.

    2. Let script be the result of fetch a module script tree given resolvedModuleURL, "anonymous" for the CORS setting attribute, and settingsObject.

    Note: Worklets follow web workers here in not allowing "use-credientials" for fetching resources.

    1. Run a module script given script.

2.1.3. Script settings for worklets

When a user agent is to set up a worklet environment settings object, given a executionContext, it must run the following steps:

  1. Let inheritedResponsibleBrowsingContext be the responsible browsing context specified by the incumbent settings object.

  2. Let inheritedOrigin be the origin specified by the incumbent settings object.

  3. Let inheritedAPIBaseURL be the API base URL specified by the incumbent settings object.

  4. Let workletEventLoop be a newly created event loop.

  5. Let workletGlobalScope be executionContext’s global object.

  6. Let settingsObject be a new environment settings object whose algorithms are defined as follows:

    The realm execution context

    Return executionContext.

    The global object

    Return workletGlobalScope.

    The responsible browsing context

    Return inheritedResponsibleBrowsingContext.

    The responsible event loop

    Return workletEventLoop.

    The responsible document

    Not applicable (the responsible event loop is not a browsing context event loop).

    The API URL character encoding

    Return UTF-8.

    The API base URL

    Return inheritedAPIBaseURL.

    The origin and effective script origin

    Return inheritedOrigin.

    The creation URL

    Not applicable.

    The HTTPS state

    Return workletGlobalScope’s HTTPS state.

  7. Return settingsObject.

Merge this with https://html.spec.whatwg.org/multipage/workers.html#set-up-a-worker-environment-settings-object

2.2. Worklet

The Worklet object provides the capability to import module scripts into its associated WorkletGlobalScopes. The user agent can then create classes registered on the WorkletGlobalScopes and invoke their methods.

interface Worklet {
    [NewObject] Promise<void> import(USVString moduleURL);
};

A Worklet has a worklet global scope type. This is used for creating new WorkletGlobalScope and the type must inherit from WorkletGlobalScope.

Note: As an example the worklet global scope type might be a PaintWorkletGlobalScope.

A Worklet has a list of the worklet’s WorkletGlobalScopes. Initially this list is empty; it is populated when the user agent chooses to create its WorkletGlobalScope.

A Worklet has a module responses map. This is a ordered map of module URLs to values that are a fetch responses. The map’s entries are ordered based on their insertion order. Access to this map should be thread-safe.

The module responses map exists to ensure that WorkletGlobalScopes created at different times contain the same set of script source text and have the same behaviour. The creation of additional WorkletGlobalScopes should be transparent to the author.

When the import(moduleURL) method is called on a Worklet object, the user agent must run the following steps:

  1. Let promise be a new promise.

  2. Let resolvedModuleURL be the result of parsing the moduleURL argument relative to the relevant settings object of this.

  3. If this fails, reject promise with a "SyntaxError" DOMException and abort these steps.

  4. Ensure that there is at least one WorkletGlobalScope in the worklet’s WorkletGlobalScopes. If not create a WorkletGlobalScope given the current Worklet.

    The user-agent may also create additional WorkletGlobalScopes at this time.

  5. Let outsideSettings be the relevant settings object of this.

  6. Run the following steps in parallel:

    1. For each WorkletGlobalScope in the worklet’s WorkletGlobalScopes, run these substeps:

      1. Let insideSettings be the WorkletGlobalScope's associated environment settings object.

      2. Fetch a module script tree given resolvedModuleURL, "omit", the empty string (as no cryptographic nonce is present for worklets), "not parser-inserted", "script", outsideSettings, and insideSettings.

        To perform the request given request, perform the following steps:

        1. Let cache be the current Worklet's module responses map.

        2. Let url be request’s url.

        3. If cache contains an entry with key url whose value is "fetching", wait (in parallel) until that entry’s value changes, then proceed to the next step.

        4. If cache contains an entry with key url, asynchronously complete this algorithm with that entry’s value, and abort these steps.

        5. Create an entry in cache with key url and value "fetching".

        6. Fetch request.

        7. Let response be the result of fetch when it asynchronously completes.

        8. Set the value of the entry in cache whose key is url to response, and asynchronously complete this algorithm with response.

      3. Let script be the result of fetch a module script tree when it asynchronously completes.

      4. Run a module script given script.

    2. If all the steps above succeeded (in particular, if all of the scripts parsed and loaded into the global scopes), resolve promise.
      Otherwise, reject promise.

    Note: Specifically, if a script fails to parse or fails to load over the network, it should reject the promise; if the script throws an error while first evaluating the promise should resolve as a classes may have been registered correctly.

  7. Return promise.

Need ability to load code into a WorkletGlobalScope declaratively. <https://github.com/w3c/css-houdini-drafts/issues/47>

2.3. Lifetime of the Worklet

The lifetime of a Worklet is tied to the object it belongs to, for example the Window.

The lifetime of a WorkletGlobalScope should be defined by subsequent specifications which inherit from WorkletGlobalScope.

Subsequent specifications may define that a WorkletGlobalScope can be terminated at any time particularly if there are no pending operations, or detects abnormal operation such as infinite loops and callbacks exceeding imposed time limits.

3. Security Considerations

Need to decide if to allow worklets for unsecure context, etc. <https://github.com/w3c/css-houdini-drafts/issues/92>

4. Examples

This section is not normative.

For these examples we’ll use a fake worklet on window.

partial interface Window {
  [SameObject] readonly attribute Worklet fakeWorklet1;
  [SameObject] readonly attribute Worklet fakeWorklet2;
};
callback Function = any (any... arguments);

[Global=(Worklet,FakeWorklet),Exposed=FakeWorklet]
interface FakeWorkletGlobalScope : WorkletGlobalScope {
    void registerAnArbitaryClass(DOMString type, Function classConstructor);
};

Each FakeWorkletGlobalScope has a map of the registered class constructors map.

When the registerAnArbitaryClass(type, classConstructor) method is called, the user agent will add the classConstructor of type to the map of registered class constructors map.

4.1. Loading scripts into a worklet.

window.fakeWorklet1.import('script1.js');
window.fakeWorklet1.import('script2.js');

// Assuming no other calls to fakeWorklet1 valid script loading orderings are:
// 1. 'script1.js', 'script2.js'
// 2. 'script2.js', 'script1.js'

4.2. Loading scripts into multiple worklets.

Promise.all([
    window.fakeWorklet1.import('script1.js'),
    window.fakeWorklet2.import('script2.js')
]).then(function() {
    // Both scripts now have loaded code, can do a task which relies on this.
});

4.3. Create a registered class and invoke a method.

// Inside FakeWorkletGlobalScope
registerAnArbitaryClass('key', class FooClass {
    process(arg) {
        return !arg;
    }
});

As an example, if the user agent wants to invoke "process" on a new class instance, the user agent could follow the following steps:

  1. Let workletGlobalScope be a FakeWorkletGlobalScope from the list of worklet’s WorkletGlobalScopes from the fake Worklet.

    The user agent may also create a WorkletGlobalScope given the fake Worklet and use that.

  2. Let classCtor be the result of performing a lookup in registered class constructors map with "key" as the key.

  3. Let classInstance be the result of Construct(classCtor).

  4. Let result be the result of Invoke(O=classInstance, P="process", Arguments=["true"]).

  5. Return result.

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
renderer
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-SYNTAX-3]
Tab Atkins Jr.; Simon Sapin. CSS Syntax Module Level 3. URL: http://dev.w3.org/csswg/css-syntax/
[HTML]
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[URL]
Anne van Kesteren. URL Standard. Living Standard. URL: https://url.spec.whatwg.org/
[WebIDL]
Cameron McCormack; Boris Zbarsky; Tobie Langel. Web IDL. URL: https://heycam.github.io/webidl/

Informative References

[CSS-PAINT-API-1]
Shane Stephens; Ian Kilpatrick; Dean Jackson. CSS Painting API Level 1. URL: https://drafts.css-houdini.org/css-paint-api-1/

IDL Index

[Exposed=Worklet]
interface WorkletGlobalScope {
    attribute Console console;
};

interface Worklet {
    [NewObject] Promise<void> import(USVString moduleURL);
};

partial interface Window {
  [SameObject] readonly attribute Worklet fakeWorklet1;
  [SameObject] readonly attribute Worklet fakeWorklet2;
};

callback Function = any (any... arguments);

[Global=(Worklet,FakeWorklet),Exposed=FakeWorklet]
interface FakeWorkletGlobalScope : WorkletGlobalScope {
    void registerAnArbitaryClass(DOMString type, Function classConstructor);
};

Issues Index

Merge this with https://html.spec.whatwg.org/multipage/workers.html#set-up-a-worker-environment-settings-object
Need ability to load code into a WorkletGlobalScope declaratively. <https://github.com/w3c/css-houdini-drafts/issues/47>
Need to decide if to allow worklets for unsecure context, etc. <https://github.com/w3c/css-houdini-drafts/issues/92>