Input and resource policy

comins-capture@0.0.1

The target must be a positive-size HTMLElement connected to the current document. Detached nodes, other documents, and whole body/documentElement targets are rejected. Finish rendering, image decode, and framework flushes before capture; keep the target stable until capture completes.

Supported input boundary

Ordinary static text, flex, table, IMG, CSS/pseudo images, and declared web fonts are processed within the validation boundary. This is not a guarantee for every CSS property or element.

Content	Contract
IMG / CSS / pseudo images	Loaded image and supported PNG/JPEG/WebP bytes
Web fonts	Readable font-face declarations and supported file bytes
SVG image resources	Supply a consumer-validated raster Blob through resolve
Forms, video, canvas, iframe, inline SVG, custom elements, open Shadow DOM	UNSUPPORTED_CONTENT
fixed/sticky, moved scroll state, backdrop-filter, active animation	UNSUPPORTED_CONTENT
Capture-root transform/zoom, vertical writing	UNSUPPORTED_CONTENT
Closed Shadow DOM	Unsupported; platform detection cannot be guaranteed

Network policy

Defaults are network:'deny' and credentials:'omit'. Already visible assets may still need bytes for capture. This policy does not control the application’s own requests or side effects performed by its resolver.

const snapshot = await capture(target, {
  resources:{network:'same-origin'},
});
try {
  const {value} = await snapshot.toBlob({format:'png'});
} finally {
  snapshot.dispose();
}

Alternatively allow exact origins with network:{origins:['https://assets.example.com']}. Paths, queries and wildcards are not allowlist entries. CORS, CSP, response and decode checks still apply; redirects are rejected. credentials:'same-origin' never sends credentials to cross-origin resources.

Resolver ownership

async function captureWithResources(target, cache) {
  return capture(target, {
    resources:{
      network:'deny',
      resolve:async ({url, signal}) => {
        if (signal.aborted) throw signal.reason;
        return cache.get(url);
      },
    },
  });
}

Here cache is an application-populated Map<string, Blob> keyed by absolute URL. Resolve receives {resourceRef, kind, url, signal} with image/font kind, and returns Blob or undefined. Undefined applies network policy; it does not skip the resource successfully. Honor signal in any asynchronous work. The caller must dispose the returned snapshot.

Unreadable stylesheets, unverifiable FontFace declarations, and local() font sources are rejected. SYSTEM_FONT diagnostics flag environment-dependent system/fallback fonts; matching glyphs across browsers are not guaranteed.

Limits and validation

Resource	Limit
One prepared resource	16,000,000 bytes
All prepared resources	64,000,000 bytes
Input image / output Canvas	8192 px per side; 32,000,000 px total
Inspected HTML nodes	10,000

Being within a numeric limit does not guarantee memory availability or supported content. SVG validation rejects scripts, event handlers, external references, DOCTYPE/entities, and unverifiable CSS. It is not a general-purpose sanitizer for arbitrary SVG input. See errors.

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Runnable example · Source guide for this version