Web platform support

What a page running in Pyro can use: JavaScript language features, DOM and HTML, CSS and layout, networking, storage, workers, media and the other Web APIs, with the known gaps. Pyro follows headless Chrome where Chrome and the specification differ.

This page summarizes the repository's own records (docs/PARITY.md, docs/COMPATIBILITY.md, docs/webapi/*, docs/browser/*). Where those records are older than the code, the presence of an interface was checked against the published 0.0.33 browser, and the newer result is given. For the CDP protocol surface see CDP coverage; for the Playwright API surface see API coverage.

Status values

Status	Meaning
Yes	Implemented and tested; gaps are edge cases.
Partial	A useful subset; the notes name what is missing.
No	Not available (the global is undefined or the feature does nothing).

Web platform tests

A pinned subset of web-platform-tests runs on every pull request in CI (.github/workflows/wpt.yml, scripts in bench/wpt). The gate fails a pull request that loses a passing subtest or breaks a page that completed. The numbers below come from the committed baseline (bench/wpt/baseline.json, WPT revision a421f2fe, last updated 2026-11-10). They count the pages the baseline records (pages that pass at least one subtest or complete), tested on the fork build (patched JavaScriptCore).

WPT directory	Pages	Subtests passing
dom	625	59,689 / 60,737 (98.3%)
html/dom	246	64,067 / 65,412 (97.9%)
css/selectors	228	5,158 / 5,803 (88.9%)
shadow-dom	185	8,700 / 8,877 (98.0%)
encoding	1,258	1,139,158 / 1,139,218 (100.0%)
url	48	9,990 / 9,999 (99.9%)
xhr	146	429 / 469 (91.5%)
fetch/api/basic	9	90 / 96 (93.8%)
uievents	17	115 / 116 (99.1%)
webstorage	51	1,271 / 1,288 (98.7%)

The subset does not cover every area on this page; areas outside it are tested by the repository's own suites.

JavaScript engine

Pages run on JavaScriptCore (the WebKit/Safari engine), so the language is Safari's: ES2025+, Intl, Temporal, WebAssembly, WeakRef/FinalizationRegistry.

Two engine builds exist (see Platforms): the patched JavaScriptCore (macOS arm64 and both Linux builds) and the system framework (macOS Intel). Features marked "patched" below need the first.

Feature	Status	Notes
ES modules	Yes	Static and dynamic import, top-level await, import.meta.url, modulepreload, SRI integrity on module scripts. Module responses must have a JavaScript, JSON or CSS MIME type.
Import maps	Partial (patched)	imports, scopes, trailing-slash packages, merging later maps, bare specifiers, import.meta.resolve. Only parser-inserted inline maps; maps added by script or document.write and external maps are ignored; integrity in a map is not enforced; a rejected map fires no error event. The system framework ignores import maps with a console warning.
JSON and CSS module scripts	Yes (patched)	with { type: "json" } and with { type: "css" } (a constructed CSSStyleSheet); a typed import fails with a TypeError on the system framework. CSS module url()s resolve against the document, not the module.
Timers	Yes	setTimeout/setInterval with the HTML 4 ms nesting clamp (windows only, not workers), queueMicrotask, requestAnimationFrame, requestIdleCallback.
unhandledrejection	Yes	The event fires (checked on 0.0.33; older records say it did not).
Console	Partial	Levels, count, time*; format specifiers such as %s are not applied.
Function.prototype.toString on interface objects	Differs	Reports function () { [native code] }.
Debugger/Profiler (inspector)	No

Runtime deviations

Deliberate, tested deviations (from docs/COMPATIBILITY.md):

Deviation	Effect
Window global shape	window is JSC's default global: Window members are own properties and Object.getPrototypeOf(window) is Object.prototype. window instanceof Window still holds and the tag is [object Window]. Patching EventTarget.prototype.addEventListener does not change window.addEventListener.
Interface objects are Proxies	Node, HTMLElement and the other interface objects are Proxies whose members are installed on first use. Descriptors and inheritance match; property operations on interface objects (including instanceof) pay a proxy-trap cost.
Termination	Code that catches a termination (Runtime.terminateExecution, a timeout) inside native frames is terminated again within about 50 ms.
Inline module URLs	Inline module scripts get import.meta.url = <document URL>#inline-module-N.
Deferred modules and DOMContentLoaded	DOMContentLoaded waits for each deferred module to finish evaluating, including top-level await. Browsers wait only for the fetch.
Root-relative module URLs (system framework)	import "/x.js" loads from the page origin but reports import.meta.url as file:///x.js.
Named window properties	window.foo finds elements by id/name (patched only), but the names are not listed by Object.getOwnPropertyNames.

DOM and HTML

Area	Status	Notes
DOM tree, Node, Element, Document, Attr	Yes	Mutation records, templates, Attr as a Node. Character data is UTF-8: lone UTF-16 surrogates are not preserved.
HTML parser	Yes	html5ever, incremental around scripts; document.write from parser-reached scripts parses at the insertion point (writes from async, deferred or module scripts are ignored).
Selectors	Yes	Stylo's matcher, :has() with state invalidation, :state(), :focus-visible, :user-valid/:user-invalid.
Custom elements	Partial	define, upgrades, all lifecycle callbacks, form-associated elements with ElementInternals, CustomStateSet. No customized built-ins (is=), no formStateRestoreCallback.
Shadow DOM	Yes	Declarative roots, getHTML, setHTMLUnsafe, Document.parseHTMLUnsafe, named and manual slot assignment, delegatesFocus.
Range, StaticRange, Selection	Partial	Range per the DOM spec; StaticRange exists; AbstractRange does not; Selection is not backed by a live Range.
NodeIterator, TreeWalker	Yes
Collections	Partial	HTMLCollection, NodeList, NamedNodeMap, DOMTokenList follow the legacy platform object rules where the JSC C API allows; index descriptors report enumerable: false and defineProperty on an index does not throw.
HTML attribute reflection	Yes	All reflection kinds of WPT html/dom/reflection-*, obsolete elements included. Accessors only: <track> and <frame> load nothing.
Node.moveBefore	Yes	Preserves focus, iframes, modal dialogs and popovers.
Tables	Partial	insertRow/deleteRow/insertCell/deleteCell and section helpers; the caption/tHead/tFoot setters are missing.
XPath	Yes	XPath 1.0, all axes and the 27 core functions, document.evaluate, XPathEvaluator/XPathExpression/XPathResult; 32-level nesting cap.
DOMParser, XMLSerializer	Partial	HTML and bounded XML 1.0 with namespaces; DTDs and XML 1.1 return a parsererror document.
XML documents, XSLT	Partial / No	XML documents load and stop at each script (0.0.31); XSLTProcessor is undefined.

Events and input

Area	Status	Notes
Event dispatch	Yes	Activation behaviour, retargeting across shadow trees, window.event.
Event handler attributes	Yes	on* IDL attributes and setAttribute('on…'); markup handlers such as <button onclick="…"> set through innerHTML run (checked on 0.0.33; older records say parser-created handlers were not compiled).
Mouse, keyboard, wheel, touch	Yes	MouseEvent, PointerEvent, KeyboardEvent, TouchEvent, DragEvent, pointer capture, Chrome's touch compatibility events. Input usually comes from CDP or the SDK.
Focus	Yes	Tab order over shadow trees and slots, positive tabindex, delegatesFocus, focus scrolling, :focus-visible.
Clipboard and editing	Partial	Typing, deletion, caret and word movement with platform modifiers, Selection.modify(), multi-click selection, copy/cut/paste with trusted ClipboardEvent, execCommand and queryCommand*, undo/redo, caretRangeFromPoint/caretPositionFromPoint, navigator.clipboard (in memory per context). Lines are logical (no soft wraps); execCommand('paste') is unavailable to script, as in Chromium.

Forms

Area	Status	Notes
Value sanitization, valueAsNumber/valueAsDate, stepUp/stepDown	Yes	All input types, including date/time kinds.
Constraint validation	Yes	Every ValidityState flag, Chrome's messages for common cases, interactive validation, :user-valid/:user-invalid, :in-range/:out-of-range.
Submission	Yes	FormData (with submitter), the formdata event, method=dialog, implicit submission on Enter, legacy encodings and accept-charset. POST to a new window fails explicitly.
Keyboard defaults	Partial	Number, range, radio groups, single <select> (one-character typeahead). No UI pickers, no multi-character typeahead.
File inputs	Yes	Set through CDP/SDK uploads; FileList, File, FileReader.

CSS, layout and rendering

Styles are computed by Stylo (Firefox's style engine) and laid out by Blitz and Taffy (block, inline, flex, grid, tables). Screenshots and PDFs are painted on the CPU. See Rendering.

Area	Status	Notes
Cascade, computed style	Yes	getComputedStyle including pseudo-elements, custom properties, matchMedia, CSS.supports.
Layout geometry	Yes	getBoundingClientRect, offset*, client*, scrolling, scrollIntoView through nested scrollers, hit testing for input.
innerText	Yes	Follows computed display (blockification, tables, display: contents).
CSSOM	Partial	cssRules parses on first access; CSSStyleRule exists. The media and disabled CSSOM APIs are missing and cssRules is a live array, not a real CSSRuleList (per docs/browser/STYLESHEETS.md). Constructed stylesheets and adoptedStyleSheets work.
Linked stylesheets	Yes	Load with load/error events, block parser scripts and load, SRI enforced. Per document: 128 stylesheets, 4 MiB per response, 16 MiB total source.
Web fonts	Partial	@font-face with unicode-range selection (0.0.31). FontFace and document.fonts' FontFaceSet constructor are undefined; document.fonts exists.
Web Animations	Partial	Element.animate, Animation and KeyframeEffect exist; completeness is not recorded.
Top layer	Yes	Modal dialogs and popovers paint above the document with ::backdrop.
visualViewport	Yes	Present on 0.0.33. Explicit screen orientation and MediaQueryList change events are missing.

Networking

Area	Status	Notes
HTTP/1.1 and HTTP/2	Yes	rustls with Mozilla's roots; HTTP/2 by ALPN (one connection per origin and context, 100 concurrent streams). No HTTP/3, no cleartext h2c, no server push.
fetch, Request, Response, Headers	Yes	Streaming bodies, abort, keepalive, referrer/referrerPolicy, SRI. Over HTTP/2 statusText is empty. The Request constructor works on 0.0.33 (older records list it as missing).
XMLHttpRequest	Yes	Sync and async, upload and download progress, responseXML, timeouts. Synchronous requests skip Fetch interception (Playwright routes do not see them).
WebSocket	Yes	Windows and dedicated workers, permessage-deflate, WebSockets over HTTP/2 (RFC 8441). Visible to Playwright's page.on('websocket').
EventSource, navigator.sendBeacon	Yes
Cookies	Yes	RFC 6265bis parsing, public suffix list, per-context jar. CookieStore is undefined.
CORS, Referrer Policy, Fetch Metadata	Yes	CORS with preflight, all eight referrer policies (default strict-origin-when-cross-origin), Sec-Fetch-* headers. Full Fetch response filtering is not claimed.
HTTP cache	Yes	Private in-memory cache per context (RFC 9111 subset, Fetch cache modes). Controlled by CDP Network.setCacheDisabled and Network.clearBrowserCache.
Authentication	Partial	Opt-in Basic challenges for servers and proxies; no credential cache or auth UI.
Proxies	Partial	HTTP forward proxy per context or serve --proxy (CONNECT for https). SOCKS and https:// proxies are refused.
file: URLs	Yes	Navigations and the subresources of file: documents; not reachable from http(s) pages or through fetch/XHR. LIGHTNINGPANDA_FILE_URLS=0 turns them off.
blob: and data: URLs	Yes	blob: loads through fetch/XHR and as script, stylesheet, image and iframe source.
URL, URLSearchParams	Yes	Current WHATWG URL Standard; every case of WPT urltestdata.json, setters_tests.json and toascii.json passes. URLPattern is undefined.
Encoding	Yes	TextEncoder/TextDecoder (streaming), legacy encodings for forms.
Downloads	Partial	With CDP Browser.setDownloadBehavior (or the SDK download step): <a download> links to blob:, data: or same-origin URLs, and navigations answered with Content-Disposition: attachment, are saved with Chrome's download events. Browser.cancelDownload is missing. See CDP coverage.
CSP, cross-origin isolation	Partial	Content-Security-Policy is only honoured for hiding the nonce attribute of script-inserted elements (no other CSP enforcement is recorded). crossOriginIsolated is always false (no COOP/COEP).

Storage

All storage is in memory: nothing is written to disk, and a new browser context starts empty unless seeded (see serve --local-storage and Profiles).

Area	Status	Notes
localStorage, sessionStorage	Yes	Per context and origin (local) and per top-level page (session); per-origin quota. No cross-document storage events. Opaque origins get per-document ephemeral storage instead of Chrome's SecurityError.
IndexedDB	Yes	IndexedDB 3.0 with getAll(options)/getAllRecords, per context and origin, shared with dedicated workers. Blob/File values throw DataCloneError; opaque origins throw SecurityError. Works with Dexie, idb, localForage, Firebase.
Cache Storage	Yes	caches per context and origin, shared by documents and workers; requires a non-opaque origin.
navigator.storage	Partial	estimate() (fixed 10 GiB quota), persisted()/persist() resolve false; no getDirectory().
Web Locks, File System Access	No	navigator.locks and showOpenFilePicker are undefined.

Workers and messaging

Area	Status	Notes
Dedicated workers	Yes	Own thread and VM; importScripts, fetch, WebSocket, IndexedDB, streams, crypto, OffscreenCanvas, transfers both ways; visible as CDP worker targets. Module workers need the patched JavaScriptCore. Worker timers are not clamped.
Service workers	Partial	Registration per context in memory, install/activate, skipWaiting, clients.claim, fetch events for page fetches, XHR, subresources and navigations, Cache Storage, MSW 2.x browser mode. Missing: requests of workers a controlled page starts, redirects answered by the worker for navigations, streamed worker responses, navigation preload, Push/Sync, persistence. navigator.serviceWorker exists only in secure contexts.
Shared workers	No	SharedWorker is undefined.
MessageChannel, postMessage	Yes	Across frames, popups and workers, with transfer of ports, streams, ImageBitmap, OffscreenCanvas. No port close event.
BroadcastChannel	Yes	Across the pages of a browser context.
structuredClone	Yes	Ordinary graphs, Error/DOMException, Blob/File, CryptoKey, ImageBitmap, transferables.
Streams	Yes	Readable (default and byte), Writable, Transform, text and compression streams (gzip, deflate, deflate-raw), transferable.

Crypto

Area	Status	Notes
crypto.getRandomValues, randomUUID	Yes
crypto.subtle	Yes	Digest, HMAC, AES-GCM/CBC/CTR/KW, PBKDF2, HKDF, ECDSA/ECDH (P-256/384/521), Ed25519/X25519, RSASSA-PKCS1-v1_5/RSA-PSS/RSA-OAEP; raw/spki/pkcs8/jwk; wrap/unwrap. No Ed448/X448. Exposed regardless of secure context.

Canvas, images and media

Area	Status	Notes
Canvas 2D	Yes	Rasterized on the CPU: paths, gradients, patterns, composite operators, shadows, text, drawImage, image data, toDataURL/toBlob (PNG/JPEG/WebP), Path2D, DOMMatrix, ImageData. Canvas filter and SVG image sources are missing. Canvases appear in screenshots.
OffscreenCanvas, ImageBitmap	Yes	Also in workers; transferControlToOffscreen.
WebGL	No	WebGLRenderingContext is undefined.
Images	Yes	<img> loading, decode(), new Image().
<audio>, <video>	Partial	Container metadata (MP4, WebM, Ogg, WAV, MP3, FLAC, ADTS), ready-state events, playback on the page clock, Chrome's autoplay policy. Video frames decode on demand for screenshots and drawImage (AV1, H.264; no VP8/VP9/HEVC). No audio output. MediaSource exists but isTypeSupported('video/mp4') returns false.
Web Audio	Partial	AudioContext (a clock, no output), OfflineAudioContext with buffer source, oscillator, constant source and gain nodes, decodeAudioData (MP3, AAC, Vorbis, Opus, FLAC, WAV). No other nodes.
MediaRecorder, WebRTC, speech	No	MediaRecorder, RTCPeerConnection and speechSynthesis are undefined.

Observers and timing

Area	Status	Notes
MutationObserver	Yes	All options, microtask delivery.
IntersectionObserver	Partial	Thresholds, root, rootMargin, clipping and scrolling; some options throw NotSupportedError. Records describe this as computed over an approximate layout.
ResizeObserver	Partial	One size for all boxes (the box option is ignored, as in upstream Lightpanda).
PerformanceObserver, timeline	Yes	User Timing, Paint Timing (first-paint/first-contentful-paint at the first rendering opportunity), Resource Timing, Navigation Timing, performance.timing.

Navigation, frames and windows

Area	Status	Notes
Location and History	Yes	Every Location setter, pushState/replaceState, popstate and hashchange in Chrome's order, <meta http-equiv=refresh> and the Refresh header.
Navigation API	Yes	window.navigation, navigate events with intercept(), entries, traversal promises.
Page lifecycle	Yes	pagehide, visibilitychange, unload when a document is left; beforeunload on Page.close.
iframes	Partial	Same-process child frames for src, srcdoc and about:blank, sandbox flags, per-frame realms, cross-origin WindowProxy restrictions, postMessage between frames. At most 128 child frames per page and 32 levels deep. No out-of-process frames, no joint session history, no loading=lazy.
window.open, popups	Yes	Opens a page in the opener's context with window.opener, named reuse and window.close(); Playwright's popup event fires.
Dialogs (alert, confirm, prompt)	Yes	Block for Page.handleJavaScriptDialog while a CDP session has Page enabled; otherwise dismissed and reported.
<dialog>, popovers, invokers, <details>	Yes	Modal dialogs and inertness, popover stacks with light dismiss, command/commandfor, exclusive accordions.

Identity and environment

Area	Default	Notes
User agent	Mozilla/5.0 (compatible; lightningpanda/<version>)	Override per page through CDP Emulation.setUserAgentOverride (Playwright userAgent); serve has no user-agent flag. navigator.userAgentData and low-entropy Sec-CH-UA headers follow the override's metadata. Some sites serve different assets to this UA than to Chrome's.
Viewport	1280x720 at DPR 1 over CDP (observed with the SDK on 0.0.33)	Set per page through CDP Emulation.setDeviceMetricsOverride or the SDK/Playwright viewport. A size change fires resize.
Locale	Process locale	navigator.language and Accept-Language follow the per-page locale; Intl and Date keep the process locale.
Timezone	Process timezone	One timezone per connection: a different zone in a second context of the same connection fails.
Permissions	Prompt	Per context through Browser.grantPermissions; Notification shows nothing (fires show when granted). navigator.geolocation exists; geolocation emulation is not recorded.
Secure context	Computed	https:, localhost, loopback, file:, about:blank, data: are potentially trustworthy.

Not available

Checked as undefined on 0.0.33: SharedWorker, WebGLRenderingContext/WebGL2RenderingContext, RTCPeerConnection, MediaRecorder, speechSynthesis, XSLTProcessor, URLPattern, CookieStore, WebTransport, FontFace, FontFaceSet, AbstractRange, TrustedHTML, Sanitizer, TextTrack, Gamepad, EyeDropper, navigator.locks. Also absent: WebDriver BiDi, Push and Background Sync, disk persistence of any storage.

If a page needs one of these, check Troubleshooting and Platforms and limits.