Supported Web Features

Overview of supported web features and browser APIs in Ultralight.

On this page

WebKit / Safari Version
HTML
CSS
Fonts and Images
JavaScript
Canvas and Graphics
Media
Networking
Storage and Crypto
Forms and Input
Clipboard
The Window and Devices
Checking for a Feature

Ultralight is a custom fork of WebKit (really, just the "WebCore" and "JavaScriptCore" modules) that began in 2014.

The latest version (2.0) is based off a build of WebKit from 2023, and most modern web features work as you would expect in recent Safari versions.

WebKit / Safari Version

Ultralight 2.0 tracks WebKit 615.1.18 (Safari 16.4). The version is readable at runtime through WebKitVersionString().

Each row below marks a feature Supported, Partial (it works with a caveat in the notes), or Not available.

HTML

Feature	Status	Notes
HTML5 parsing and elements	Supported	Works as it does in Safari.
Web components (custom elements, shadow DOM, `<slot>`)	Supported
`<dialog>` and `<details>`	Supported
`contenteditable`	Supported
`<input type="file">`	Partial	Opens no picker (`File` and `FileReader` work).
`loading="lazy"` on images	Not available	Ignored, so every image loads right away.
The `popover` attribute	Not available	Arrived after this WebKit generation.
MathML	Not available
Date, time, month, week, and color inputs	Not available	They act as plain text inputs.
`<datalist>`	Not available	Shows nothing.
Spell checking	Not available	`spellcheck` has no effect.

CSS

Feature	Status	Notes
Flexbox	Supported
Grid and subgrid	Supported
Custom properties	Supported
Container queries	Supported
Cascade layers	Supported
Nesting	Supported
`:has()`	Supported
Filters and backdrop filters	Supported
Blend modes	Supported
`image-rendering: pixelated` and `crisp-edges`	Supported	Scaled images and pixel art keep hard pixel edges instead of being smoothed.
Transitions and animations	Supported
`prefers-color-scheme`	Supported	Follows the scheme your app reports (see Dark Mode and Color Schemes).
`(hover: ...)` and `(pointer: ...)` media queries	Partial	They report no pointing device at all: `(hover: hover)` and `(pointer: fine)` never match, `(hover: none)` and `(pointer: none)` do. Don't gate a desktop layout on them— a responsive framework can end up serving its touch layout.
Image cursors (`cursor: url(...)`)	Partial	Ignored, though named cursors reach your app.
Smooth scrolling	Not available	`scroll-behavior: smooth` and the `behavior: 'smooth'` option jump straight to the target.
`hyphens: auto`	Not available	Never hyphenates.
View transitions	Not available	Arrived after this WebKit generation.
Anchor positioning	Not available	Arrived after this WebKit generation.
`@scope`	Not available	Arrived after this WebKit generation.
`text-wrap: balance`	Not available	Arrived after this WebKit generation.

Fonts and Images

Feature	Status	Notes
`@font-face` web fonts	Supported	TrueType, OpenType, WOFF, and WOFF2.
Variable fonts	Supported	`font-weight`, `font-stretch`, `font-style`, and `font-variation-settings`.
PNG, JPEG, GIF, WebP, BMP, and ICO images	Supported
SVG	Supported	Inline and as an image.
`.dds` textures in `<img>` and CSS	Supported	See Compressed Textures.
Textures straight from your app	Supported	See Displaying Custom Textures.
AVIF and JPEG XL images	Not available	They don't decode.

JavaScript

The Free edition has JIT disabled (JavaScript may execute slower). The editions are covered in Editions and Feature Macros.

Feature	Status	Notes
ES2022	Supported
The ES2023 array additions	Supported
ES modules and dynamic `import()`	Supported
Web Workers	Supported
Secure contexts on local pages	Supported	Pages loaded from `file://` URLs are secure contexts, and so is HTML loaded from a string with a `file://` base URL. Without one, that HTML is `about:blank`, which is not, and secure-context-only APIs such as `crypto.randomUUID()` and `navigator.locks` disappear— so pass a base URL.
Switching JavaScript off per View	Supported	The page still lays out and animates, and your app drives it from native code (see Embedded and Device UI).
`Object.groupBy`, the RegExp `v` flag, `Temporal`	Not available	Anything later than the Safari 16.4 level.
WebAssembly	Not available	Pick the JavaScript build of any library that offers one.
`SharedArrayBuffer`	Not available
Shared workers	Not available
Service workers	Not available
`requestIdleCallback()`	Not available	Schedule background work with `setTimeout()` or `requestAnimationFrame()`.

Canvas and Graphics

Feature	Status	Notes
Canvas 2D	Supported	Including `Path2D`, `roundRect()`, conic gradients, `ImageBitmap`, and `toBlob()`.
`imageSmoothingEnabled` and `imageSmoothingQuality`	Supported	Turning smoothing off keeps hard pixel edges when scaling. `'high'` applies a higher-quality filter when enlarging. `'low'` and `'medium'` use the default.
`OffscreenCanvas`	Supported	On the page and in workers.
The canvas `filter` property	Partial	Works unless your app switches canvas filters off. Reference filters like `url(#id)` draw unfiltered.
Canvas readback on a GPU-rendered View	Partial	`getImageData()` throws, `toDataURL()` returns `data:,`, and `putImageData()` does nothing. Create the context with `getContext('2d', { willReadFrequently: true })` and all three work. CPU-rendered Views are unaffected.
WebGL	Not available	`getContext('webgl')` returns `null`. Draw 3D content in your engine and show it on the page as a custom texture (see Displaying Custom Textures).
WebGPU	Not available

Media

Feature	Status	Notes
HTML5 video and audio	Partial	Pro edition and higher, from the WebM container with VP9 video and Opus audio (in lower editions media playback isn't available). See Media and Audio.
`autoplay`	Supported	Works without a user gesture.
The `controls` attribute	Supported
`<track>` subtitles	Supported
Media Source Extensions	Not available	An HLS or DASH player won't work— point the element at a WebM file instead.
Web Audio	Not available
WebRTC and `getUserMedia()`	Not available
DRM (Encrypted Media)	Not available
Picture-in-picture	Not available
WebCodecs	Not available

Networking

Feature	Status	Notes
`fetch()` and `XMLHttpRequest`	Supported	Requests go over HTTP/1.1 or HTTP/2.
Streaming response bodies	Supported
`EventSource`	Supported
`WebSocket`	Supported
Content Security Policy	Supported	Enforced.
Downloads	Partial	A file the server sends as an attachment, or one the page can't display, reaches your app as a download (see Downloads and Network Control). An `<a download>` link just navigates.
`navigator.sendBeacon()`	Not available	Send analytics with `fetch()` while the page is alive.

Storage and Crypto

Feature	Status	Notes
Cookies	Supported	They live in the View's session (see Sessions and Site Data).
`localStorage` and `sessionStorage`	Supported	In the View's session.
IndexedDB	Supported	In the View's session.
`BroadcastChannel`	Supported
`navigator.locks`	Supported
`crypto.getRandomValues()` and `crypto.randomUUID()`	Supported
`crypto.subtle`	Not available	Hashing and signing need a JavaScript library.
The Cache API	Not available
`navigator.storage`	Not available
`CompressionStream`	Not available

Forms and Input

Feature	Status	Notes
Mouse, wheel, keyboard, and pointer events	Supported	From the input your app forwards (see Mouse and Scroll Input and Keyboard Input).
The Gamepad API	Supported	From the controllers your app forwards (see Gamepad Input).
Form validation	Partial	An invalid control blocks the submission. `invalid` fires on each bad control and the first takes focus, but no message bubble is drawn, so style `:invalid` or show your own text (see Forms and Inputs).
Touch events	Not available	Use pointer events.
HTML drag and drop	Not available	`draggable` never starts a drag. Use pointer events.

Clipboard

Feature	Status	Notes
Copy and cut	Supported	From script and the keyboard.
Script-initiated paste	Partial	Works only during a click or key press and only when your app's clipboard policy allows it (see Text Editing and the Clipboard).
Text and HTML on the clipboard	Supported
Images and files on the clipboard	Not available
`navigator.clipboard`	Not available	Use `document.execCommand()`.

The Window and Devices

Feature	Status	Notes
`devicePixelRatio`	Supported	Reflects the View's scale.
`window.screen`	Partial	Reports zero for its size, so size your layout from `innerWidth` / `innerHeight` or viewport units.
`window.open()` and `target="_blank"`	Partial	They open a new View only when your app provides one, and `window.open()` returns `null` otherwise (see Handling View Events).
`alert()`, `confirm()`, and `prompt()`	Not available	`alert()` shows nothing, `confirm()` returns `false`, and `prompt()` returns `null`. Build dialogs in the page or natively (see Popups and Dialogs).
`beforeunload` prompts	Not available	`beforeunload` never asks.
`window.print()`	Not available
`requestFullscreen()`	Not available	Your app owns the window (see Windows, Monitors, and DPI).
`requestPointerLock()`	Not available
`Notification`	Not available
`navigator.geolocation`	Not available
Permissions, Web Share, wake locks, Payment Request, Web Authentication, speech, WebXR	Not available
Assistive technology	Not available	Pages are not exposed to screen readers (ARIA attributes parse but reach nothing).

Checking for a Feature

Feature-detect the usual way, with typeof, 'x' in navigator, or CSS.supports(). A missing API is simply undefined, with no other message.

JavaScript

if ('requestIdleCallback' in window)
  requestIdleCallback(saveDraft);
else
  setTimeout(saveDraft, 0);

Supported Web Features

#WebKit / Safari Version

#HTML

#CSS

#Fonts and Images

#JavaScript

#Canvas and Graphics

#Media

#Networking

#Storage and Crypto

#Forms and Input

#Clipboard

#The Window and Devices

#Checking for a Feature

WebKit / Safari Version

HTML

CSS

Fonts and Images

JavaScript

Canvas and Graphics

Media

Networking

Storage and Crypto

Forms and Input

Clipboard

The Window and Devices

Checking for a Feature