The Tool Desk
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What html2canvas is—and why that matters
The html2canvas documentation describes a script that “traverses through the DOM of the page it is loaded on.” It does not copy the browser’s already-painted pixels. It creates its own representation from element information and implemented CSS properties. Consequently, a visually identical page can still produce different output if the DOM, computed styles, viewport, scroll offsets, loaded resources, or timing differ between calls.
This distinction also sets a limit: changing a delay or option cannot make an unsupported CSS property render correctly. Every CSS property has to be implemented by html2canvas, and the project’s FAQ says complete CSS support is not possible.
First, make the capture inputs repeatable
Hold the application state constant
Capture the same route, selected tab, expanded rows, authentication state, data response, and target element each time. A framework may still be rendering after the click that starts your capture. Wait for the application’s own “ready” signal and for the assets relevant to the target. document.fonts.ready waits for fonts, not for every image or network request.
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Fix the viewport and pixel scale
By default, scale follows window.devicePixelRatio. Two devices, monitors, browser zoom levels, or remote sessions can therefore produce different pixel dimensions. Set it explicitly when byte-for-byte or dimension-for-dimension consistency matters. Set windowWidth and windowHeight to keep media-query decisions stable.
Fix scroll coordinates
Sticky and fixed elements are positioned relative to the viewport. A different scroll position changes their appearance and can alter what html2canvas reconstructs. Set scrollX and scrollY deliberately, especially when capturing an element from a scrolled page.
Remove transient behavior in the cloned document
Use onclone to change only the document used for rendering. You can disable animations and transitions, hide blinking cursors, close menus, or remove a timestamp without disturbing the live interface.
A minimal stabilization pattern
await document.fonts?.ready;
const element = document.querySelector('#capture');
if (!element) throw new Error('Missing #capture');
const canvas = await html2canvas(element, {
scale: 1,
windowWidth: 1280,
windowHeight: 800,
scrollX: 0,
scrollY: 0,
onclone: (clonedDocument) => {
clonedDocument.querySelectorAll('.animated, .transient').forEach((node) => {
node.style.animation = 'none';
node.style.transition = 'none';
});
},
onError: (error) => console.error('Capture resource error:', error)
});
document.body.appendChild(canvas);This is a stabilization pattern, not a guarantee that every application is ready. Replace the selectors and viewport with values appropriate to your page, and wait for asynchronous data and images using signals supplied by your application.
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Diagnose differences before comparing pixels
- Use one target. Pass the same element reference or selector and log its
scrollWidth,scrollHeight,getBoundingClientRect(), and computed display state. - Log the rendering context. Record
window.innerWidth,window.innerHeight,devicePixelRatio,scrollX,scrollY, and the html2canvas options for each click. - Check readiness. Confirm that application data, fonts, and images have completed before invoking html2canvas. A loading spinner or late layout shift is enough to change the reconstruction.
- Freeze motion. Apply the
onclonechanges above. If the output becomes stable, the cause is mutable animation or transient UI state. - Keep diagnostics enabled. Do not disable logging while investigating. Use
onErrorto record resource failures and inspect the browser console and network panel.
Why aren’t my images rendered?
Images hosted on another origin are subject to the browser’s content-security rules. Set useCORS: true only when the image server sends an appropriate Access-Control-Allow-Origin response. That option requests a CORS load; it cannot grant permission that the server did not provide. If you control neither server, use a documented proxy route or serve the asset from an allowed origin.
const canvas = await html2canvas(element, {
useCORS: true,
logging: true,
onError: (error) => console.error(error)
});Check the image request itself in developer tools. A redirect to a host without CORS headers, an authentication failure, or a blocked mixed-content request can look like an html2canvas rendering defect.
Why is the produced canvas empty or cut off halfway through?
Canvas dimensions are limited by the browser, operating system, graphics hardware, and available memory. The FAQ’s dimension and area figures are rough guidance, not universal constants. A very tall full-page capture can exceed a limit and become blank or partially rendered.
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Reduce the rendering size
- Capture a smaller element or split a long page into sections.
- Lower
scaleinstead of multiplying a large CSS area by a high device-pixel ratio. - Set
windowWidthandwindowHeightto the dimensions needed for the target, rather than an accidentally huge document size. - Test the same capture in another browser or on another machine to distinguish an application problem from a platform canvas limit.
Also verify that the target is not hidden, has nonzero dimensions, and is not being clipped by an ancestor with overflow rules. Those are layout problems, not random capture behavior.
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Why doesn’t CSS property X render correctly or only partially?
html2canvas supports a defined subset of CSS. Unsupported or partially supported properties will remain unsupported regardless of timing, scale, or viewport settings. Consult the project’s supported-features reference, then isolate the suspected declaration by temporarily replacing it with a simpler equivalent. Commonly useful diagnostics include removing filters, complex blend effects, unusual generated content, or newer layout features one at a time.
If the requirement is the browser’s exact painted result—including every CSS effect—html2canvas may be the wrong method. It is a DOM reconstruction tool, not a native compositor capture.
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Viewport reports and the “different images” issue
GitHub issue #2947, opened on September 5, 2022, reports that “the screenshot taken with different viewports return different images.” The report describes Chrome 104.0.5112.102 on Windows 10 and includes an onclone customization. It is a useful example of the symptom, not proof that every varying capture has one root cause or that html2canvas is inherently random.
When you see this symptom, compare the actual viewport values, media-query branches, target dimensions, scroll offsets, and cloned-document changes. Setting windowWidth and windowHeight controls the rendering context; it does not make the source application’s data or UI state identical.
Choosing a different capture method
| Need | Better fit | Reason |
|---|---|---|
| Client-side reconstruction of a selected DOM element | html2canvas | Runs in the page and can export a canvas, subject to CSS, CORS, and canvas limits. |
| Browser extension capture of what the user sees | Native tab-capture API | The browser can capture its painted tab rather than rebuilding DOM and CSS. |
| Server-side screenshots with high browser fidelity | Puppeteer or Playwright | The official FAQ recommends a real-browser tool for server-side work. |
| Hosted API with cleanup and repeatable automation | ScreenshotNeo | It accepts a URL, handles browser automation, and reports whether a response was billed. |
Or skip the browser setup
ScreenshotNeo is a website screenshot API and MCP server for developers. It loads the URL in a browser, accepts cookie or consent banners like a visitor, and removes more than 60 known consent platforms, newsletter popups, and chat widgets before capture; each cleanup step can be disabled. Bot checks and CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and response headers identify the page verdict and billing result.
One GET request returns PNG, JPEG, WebP, or a PDF. The API supports full-page captures with lazy images, CSS-selector element captures, dark mode, device presets or custom viewports, retina scale, PDF paper and page controls, custom CSS and JavaScript, pre-capture clicks, hidden selectors, selector or network-idle waits, request and resource blocking, headers, cookies, user agents, authorization, timezone, geolocation, transparent backgrounds, resizing, chosen cache TTLs, signed image links, asynchronous jobs with signed webhooks, bulk capture of up to 100 URLs per call, usage reporting, and an OpenAPI specification. Parameter names used by other screenshot APIs also work.
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cURL
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpPython
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
r.raise_for_status()
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Common failure modes and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| Different dimensions on each click | Device-pixel ratio, zoom, or viewport changed | Set scale, windowWidth, and windowHeight. |
| Only animated regions differ | Animation, transition, caret, or rotating content | Disable it in onclone and capture a stable state. |
| Images are missing | Cross-origin response lacks permission | Configure server CORS, use useCORS, or use a proxy. |
| Output is blank or truncated | Canvas limit or excessive scale | Reduce area or scale and split long captures. |
| One CSS effect is wrong | Unsupported or partial CSS implementation | Check supported features or use a real-browser capture. |
| Capture starts before content appears | Async application work is incomplete | Wait on the application’s data and asset readiness signals. |
FAQ
Is html2canvas random?
The documented behavior is deterministic for identical inputs, but the inputs often are not identical. Treat variation as evidence of changed state, environment, resources, or an implementation limit—not as a demonstrated random-number process.
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Can html2canvas capture a cross-origin image anyway?
No. The image server must permit the request through CORS, or the image must be supplied through a suitable proxy.
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Usually not. Use a real-browser tool such as Puppeteer or Playwright, or a hosted browser screenshot API when you do not want to operate the browser infrastructure.
Frequently Asked Questions
Does calling html2canvas twice reuse the first canvas?
Each call reconstructs the target from the current DOM and rendering context. Reusing a canvas element does not freeze the page state; stabilize the inputs before every call.
Why does changing only browser zoom alter the image?
Zoom changes effective layout and often device-pixel scaling. Fix the viewport and html2canvas scale, and keep browser zoom constant when comparing outputs.
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