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Why a canvas becomes tainted
Browsers mark a canvas as not origin-clean when it contains pixel data drawn from a source the page is not allowed to read. The image may still appear on screen, but exporting its pixels with toDataURL(), toBlob(), or reading them with getImageData() can throw a SecurityError. This restriction prevents a page from extracting private data from another origin. The HTML Standard’s canvas security rules describe the export checks; MDN’s canvas image guide explains the practical effect for cross-origin images.
html2canvas does not take a native screenshot of the browser window. It walks the DOM and recreates the parts it supports on a canvas. Its result can differ from the page’s appearance where a CSS feature or embedded content is unsupported. It also cannot read a nested canvas that was already tainted before the capture began. See the project’s overview and limitations.
For a fully offline capture, localize the dependencies and load them under a consistent local origin. “The capture code runs offline” is not enough if the page still requests an online image, font, stylesheet, script, or proxy.
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Find the resource causing the problem
Check every element that contributes pixels inside the capture area—not only visible <img> elements. Background images, SVG references, web fonts, video frames, and nested canvases can matter too. An image omitted from the final result may be a clue that html2canvas skipped a resource it expected would taint the canvas.
- Check the browser console. Look for CORS messages, failed network requests, or errors involving local files. The network panel can show which assets failed to load or came from a different origin.
- Inspect the capture subtree. Search its markup and styles for image URLs, then check for nested
<canvas>, SVG, video, and externally loaded fonts. - Confirm assets are ready. Start rendering after required images and other dependencies have loaded. A missing offline asset is a load problem; it is not fixed by changing the export format.
- Separate the page-origin issue from the offline issue. A page can be offline but served from localhost, or online while opened as a
file://document. These conditions have different consequences for browser security.
The html2canvas FAQ documents same-origin and proxy considerations, cross-origin frames, and pre-tainted canvases. Cross-origin iframes are blocked by browser security restrictions; the renderer cannot use a proxy to read their contents as if they belonged to the page.
Make an offline page work from localhost
Double-clicking an HTML file opens it with a file:// URL. Modern browsers commonly treat file URLs as opaque origins, so two files in the same folder are not necessarily considered same-origin for security checks. MDN documents this behavior in its guidance on CORS requests that are not HTTP and the same-origin policy.
For local development, serve the project folder through a development server and open its localhost address. Files served using the same localhost scheme and domain share an origin. This avoids the particular ambiguity of file origins; it does not bypass CORS for unrelated remote websites. If the computer must stay disconnected, the HTML, JavaScript library, stylesheets, fonts, images, and other required files still need to be available locally.
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The project’s Getting Started guide describes browser setup and lists modern evergreen browsers including Chrome or Chromium-based browsers, Firefox, and Safari. It says the library runs in the browser rather than Node.js. Its documentation does not establish a release number for the option behavior discussed here, so check the documentation matching the copy of the library in your project.
Choose a fix for each image or embedded source
| Source or situation | Recommended action | Works fully offline? |
|---|---|---|
| Local image, font, or stylesheet | Include it with the project, serve it from the same local origin, and wait for it to load. | Yes, if all required files are already available locally. |
| Remote image server that supports CORS | Request it with useCORS: true and confirm the server sends an appropriate Access-Control-Allow-Origin header. |
No, unless the needed resource has already been made available locally. |
| Remote image server you cannot configure | Use an application-controlled proxy, or omit or replace the image. A proxy must fetch and return the asset through your application’s origin. | No for an online proxy; only if the asset and required proxy service are available locally. |
| Unneeded image or widget | Exclude it from the rendered subtree or mark the element with data-html2canvas-ignore. |
Yes; the excluded pixels will not appear. |
| Already-tainted nested canvas | Remove or replace that canvas, or recreate its contents from permitted resources before capturing. | Yes, if the replacement uses available, permitted resources. |
The configuration guide explains the available settings and defaults. A proxy adds a server dependency and should be designed for your application’s security and availability needs; it is not an offline workaround unless the necessary resources are already local. General background on the server-side mechanism is in MDN’s CORS guide.
Use CORS settings only when the server permits them
For a remote image, useCORS: true asks html2canvas to load it using a CORS-enabled image request. The image host must cooperate by returning suitable CORS headers. The setting cannot make a non-cooperating server grant permission. This is why it is usually not helpful for a genuinely offline workflow: an offline browser cannot fetch an unavailable remote image, and a cached image is not automatically authorized for canvas export.
allowTaint is easy to misread. Its default is false; in that mode, html2canvas skips resources it expects would taint the output. Setting it to true allows a cross-origin resource to be drawn even if that leaves the canvas tainted. It does not authorize pixel export. If the goal is a downloadable image, use same-origin assets, a server that supports CORS, or omit the resource. The configuration reference and FAQ explain these options.
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Render, export, and download a PNG
Assuming html2canvas is loaded and the capture element’s dependencies are ready, this example renders the selected element and downloads a PNG. The CORS option is appropriate only if the page contains cross-origin images whose servers actually grant CORS access. For a local-only page with same-origin assets, it can be omitted.
const target = document.querySelector('#capture');
if (!target) {
throw new Error('Could not find #capture');
}
const canvas = await html2canvas(target, {
useCORS: true,
});
const blob = await new Promise((resolve, reject) => {
canvas.toBlob((result) => {
if (result) {
resolve(result);
} else {
reject(new Error('Canvas export returned no blob'));
}
}, 'image/png');
});
const objectUrl = URL.createObjectURL(blob);
const link = document.createElement('a');
link.download = 'capture.png';
link.href = objectUrl;
link.click();
setTimeout(() => URL.revokeObjectURL(objectUrl), 0);The project’s examples show a toDataURL('image/png') download pattern. toBlob() is another way to produce the file, but both obey the same origin-clean rule. If either export method throws, changing serializers will not repair the canvas: identify the source that tainted it, then fix or exclude that source.
Troubleshoot errors and missing content
SecurityError from toDataURL(), toBlob(), or getImageData()
Cause: At least one drawn source tainted the canvas, or a nested canvas was tainted before html2canvas captured it. Fix: Check the resource list and network console, replace cross-origin content with local files or CORS-authorized assets, and remove or recreate pre-tainted canvases. Do not keep swapping export methods; they are all subject to the browser’s origin-clean checks.
Images disappear from the html2canvas output
Cause: The image failed to load, was unavailable offline, or was skipped because it was expected to taint the canvas. Fix: Verify its URL and load status, place the asset under the local app origin for offline work, or configure the image server for CORS and use the CORS loading option. If the image is not needed, exclude it intentionally rather than relying on a failed request.
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A local sibling file fails only when opening the HTML directly
Cause: The browser is applying its special file:// origin behavior. Fix: Serve the folder from localhost, then ensure the asset paths resolve from that server. This addresses file-origin ambiguity; it does not authorize a request to another site.
useCORS: true makes no difference
Cause: The remote host does not return a suitable CORS header, the request cannot reach it, or the failing source is something other than a CORS-enabled image. Fix: Check the actual response headers and network failure, make the resource local for offline captures, use an application-controlled proxy where appropriate, or omit the source. The browser’s policy cannot be overridden by an html2canvas option.
The export succeeds but the capture looks different
Cause: html2canvas reconstructs the DOM using the browser APIs and CSS features it supports; it is not a pixel-for-pixel browser screenshot. A missing or altered feature may be a rendering limitation rather than a canvas security error. Fix: Check the project’s documented limitations, simplify or replace unsupported styling, and compare the rendered output before troubleshooting export security.
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If an online screenshot service fits the job, ScreenshotNeo offers a website screenshot API and MCP server. It is not an offline replacement for html2canvas: the API captures a URL, so use the local workflow above when the page and assets must remain offline.
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For an online page, one GET request returns an image or PDF. For example, this cURL request saves a WebP capture of Stripe; replace the URL with the page you need. See the ScreenshotNeo documentation for request options and response details.
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpThe same request in Python:
import requests
r = requests.get(
"https://api.screenshotneo.com/v1/shot",
params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"},
timeout=90,
)
open("shot.webp", "wb").write(r.content)Or in Node.js:
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);- Cookie and consent banners, newsletter popups, and chat widgets are removed before capture; each cleanup step can be turned off.
- Bot checks or CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed; response headers report the page verdict and billing status.
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Frequently Asked Questions
Can I export an html2canvas image if I am completely offline?
Yes, if the page, library, and every resource needed for the capture are available locally and loaded from an origin the browser permits. A remote dependency that cannot be reached will not become available just because the export code is local.
Does changing the output from PNG to JPEG or WebP remove canvas taint?
No. Canvas origin-clean checks apply to export regardless of the image format.
Can I capture a cross-origin iframe with html2canvas?
Not by reading the iframe’s contents across origins. Browser security blocks access to cross-origin frame content.
Does ScreenshotNeo capture a page that exists only on my computer?
The API example captures a URL; it is not an offline capture of a local-only page. Use the local html2canvas approach when the page must remain offline.
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