If a Polymer website looks incomplete or broken in wkhtmltoimage, first check JavaScript execution, resource loading, and capture timing; then verify that the Polymer build and its Web Components polyfills match the renderer’s capabilities. wkhtmltoimage uses Qt WebKit, not a current Chrome engine, so some dynamic JavaScript or modern browser features may not behave as they do in a regular browser. If the page depends on capabilities the installed build cannot provide, test a modern browser-based renderer instead.
Why Polymer pages can render differently
wkhtmltoimage is part of the wkhtmltopdf project and renders HTML through Qt WebKit in a headless process. Polymer 3 applications use ES6 modules and modern JavaScript and Web Components features. That creates a plausible compatibility gap: a page relying on newer syntax, native modules, custom elements, Shadow DOM, or asynchronous component initialization may not render as it does in a current browser. The project and framework documentation establish those technologies, but do not establish that every Polymer version or every wkhtmltoimage build will fail in the same way. wkhtmltopdf project Polymer 3 ES6 documentation
Treat the problem as a diagnosis, not an assumption that “Polymer is unsupported.” A missing component can result from an engine limitation, a JavaScript error, an unrequested resource, a build target mismatch, or a capture that happens before the application is ready.
Start with a reproducible comparison
- Record the exact binary and command. Note the installed wkhtmltoimage build, operating environment, complete command line, target URL (or local HTML file), and output format. Wrapper programs and packaged builds can differ, so do not assume behavior from another machine’s version.
- Capture the same URL in a current browser. Compare the browser result with the generated image. Record which elements are absent, whether styles are missing, and whether the page changes after it first appears.
- Keep the evidence together. Save stderr and any available renderer logs with the command and image. This makes it easier to distinguish a load failure from unsupported runtime behavior or an early capture.
The project’s documentation describes available settings but cannot identify the version or behavior of the binary installed on your system. Check the installed build rather than copying a command-line option from a different wrapper without confirming its spelling and support.
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Check JavaScript and resource loading settings
Before changing the application, establish that the renderer is allowed to run JavaScript and load images and other required resources. The libwkhtmltox settings include controls for JavaScript, image loading, JavaScript debugging, slow scripts, and delay. Their exact flags or configuration names can vary by interface; consult the settings for the interface you use. libwkhtmltox page settings
- JavaScript: Confirm it is enabled. A Polymer app that requires JavaScript cannot render its application UI if scripts are disabled.
- Images: Confirm image loading is enabled, especially if the symptom is missing imagery rather than uninitialized components.
- JavaScript errors: Turn on the documented debugging option where available and inspect stderr or logs. Resolve failed scripts and runtime errors before interpreting the result as a rendering-engine limitation.
- Slow scripts and failed resources: Inspect the relevant settings and logs so that scripts or resources that have not completed are not silently mistaken for content the page does not contain.
Change one setting at a time and recapture the same page. A controlled comparison is more useful than enabling several options at once, because it shows which change affects the result.
Use delay carefully; it is not application readiness
A JavaScript delay waits a specified interval after page load before capture. The command-line manual documents a JavaScript delay option, and the library settings include load.jsdelay. Increasing the delay may help when a page adds content shortly after its initial load. It does not prove that network activity has stopped, that a Polymer component has upgraded, or that application-specific data is ready. libwkhtmltox page settings
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First compare captures at the current delay and at a longer delay. If the page progressively fills in, timing is likely part of the problem. If the same components remain broken after a reasonable wait, investigate errors, resource loading, build output, and runtime support instead of indefinitely increasing the delay.
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Check the Polymer build and Web Components setup
Build for the renderer’s capabilities
Polymer 3 uses JavaScript modules and ES6 features. Polymer’s build tools can transform modules and syntax for browsers with lower capabilities, but the documentation does not certify Qt WebKit as a supported target. If you control the application, inspect the deployed build and consider compiling the relevant elements, Polymer library, and third-party elements consistently for the browser capabilities you need. A transform cannot add every missing browser API, so test the actual output in the installed renderer.
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Do not test a transformed application shell while leaving an essential third-party element or dependency in a newer syntax or module format. Compatibility depends on the runtime code the renderer receives, not just the application’s top-level entry point. Polymer 3 ES6 documentation
Use polyfills and adapters in the documented order
Polymer’s Web Components documentation describes webcomponentsjs v2 polyfills for Custom Elements and Shadow DOM, along with bundle and loader approaches. Choose the setup appropriate to the Polymer generation and build you actually serve; do not mix multiple copies or incompatible generations of the polyfills. Polymer Web Components polyfills documentation
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Decide whether to keep wkhtmltoimage
The wkhtmltopdf project recommends considering Puppeteer or a wrapper when converting sites that use dynamic JavaScript. This is guidance from the project, not a guarantee that a replacement will fix every Polymer page. Validate any alternative against the target site’s actual output and deployment needs. wkhtmltopdf project status
Compare the renderers on the requirements that affect your page:
- Whether its JavaScript engine can parse the syntax and load the module format your build serves.
- Whether custom elements and Shadow DOM behave as the application expects.
- Whether you can wait for a page-specific readiness condition rather than relying only on a fixed delay.
- Whether the renderer can load required resources and handle the page’s authentication needs.
- Whether its output is reproducible in your deployment environment and fits your operational requirements.
- How it handles untrusted HTML. The wkhtmltopdf project warns against using its tool with untrusted HTML and recommends sanitizing user-supplied HTML and JavaScript.
There is no benchmark or universal feature comparison established here. Test representative pages, including the components and loading paths that matter to your own application, before changing production rendering.
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Troubleshooting by symptom
| Symptom | What to check | Next action |
|---|---|---|
| Polymer components are absent or unupgraded | JavaScript errors, module loading, custom-element support, and capture timing | Inspect logs; verify build target and polyfill setup; compare with a longer delay to determine whether initialization is merely late. |
| Page shell appears, but dynamic content is missing | Whether content arrives after initial load, whether required requests succeed, and whether scripts are enabled | Check resource and JavaScript settings. Use delay only as a timing test; seek an application readiness signal for repeatable results. |
| Images are missing while text renders | Image-loading configuration and failed image requests | Enable image loading in the interface you use and inspect resource errors. |
| Compiled code still fails around custom elements | Whether the ES5 adapter applies to this build and precedes the polyfills | Follow Polymer’s documented ordering and avoid duplicate or incompatible polyfill copies. |
| Longer waits do not change the result | Syntax or API support, module loading, runtime errors, and build consistency | Stop treating delay as the fix; test a compatible build or evaluate a current browser automation renderer. |
Or skip the browser setup
If you need screenshots rather than a local rendering pipeline, ScreenshotNeo is a website screenshot API and MCP server for developers. It offers a one-request capture and options for controlling capture behavior; it is an alternative to test, not a guarantee that every Polymer site will render identically. See the ScreenshotNeo API documentation for request options.
This cURL request captures a URL as a WebP image. Replace the example URL with your page and use your API key:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpScreenshotNeo accepts consent banners like a visitor and removes 60+ known consent platforms, newsletter popups, and chat widgets before capture; each step can be turned off. Bot checks, blank pages, timeouts, failed loads, and cache hits cost nothing, and response headers report the page verdict and billing status. Its MCP server offers take_screenshot, get_page_info, and capture_pdf for AI agents, including Claude, Cursor, and other MCP clients. The Free plan includes 1,000 screenshots a month with no card; paid plans start at $5 for 3,000.
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Does every Polymer website fail in wkhtmltoimage?
No. The compatibility risk depends on the Polymer build, page features, and installed wkhtmltoimage build; test the specific site and binary.
Will increasing the JavaScript delay always fix missing components?
No. A delay can allow late content to appear, but it does not confirm application readiness or supply unsupported browser features.
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