Use a JavaScript-capable browser, not WeasyPrint, when the page depends on a remote script. WeasyPrint can fetch remote assets, but its renderer does not execute JavaScript. With Playwright for Python, open the page, inject the script with page.add_script_tag(url=...) when necessary, wait for the application’s own ready signal, and then call page.pdf(). Playwright prints with print media by default; call page.emulate_media(media="screen") first when the screen stylesheet is the layout you need.
Why WeasyPrint cannot load and run a JavaScript URL
Fetching a file and executing it are different operations. WeasyPrint’s Python API and default fetcher can retrieve network resources such as stylesheets, images and fonts, but the HTML-to-PDF renderer does not run page JavaScript. A script loaded with a <script src="..."> element therefore cannot build a chart, request application data, or alter the DOM before WeasyPrint creates the PDF.
That limitation is intentional: WeasyPrint is designed for static HTML and CSS rendering. Its documentation describes a model with no user interaction, JavaScript or live rendering after the document is parsed. If the final HTML already contains all content and only needs CSS layout, WeasyPrint remains a good fit. If content appears only after JavaScript executes, render the page in a browser first.
Choose the renderer that matches the page
| Requirement | Recommended approach | Reason |
|---|---|---|
| Static or mostly static HTML/CSS | WeasyPrint | Its Python API converts HTML to PDF and can fetch referenced resources, but it does not execute JavaScript. |
| Data or layout created by JavaScript | Playwright for Python | A real browser runs the script, performs client-side requests and exposes page.pdf(). |
| PDF/A output | Verify the renderer and profile before implementation | PDF/A variants prohibit JavaScript embedded as active PDF content. Running JavaScript in a browser before producing a PDF is a separate issue, but the output requirements still need checking. |
Make the decision from four questions: does the source require JavaScript, how closely must browser layout be reproduced, when is the application actually ready, and which network or filesystem resources may the renderer access?
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Install Playwright for Python
- Create or activate a virtual environment.
- Install the Python package:
pip install playwright. - Install a browser engine:
playwright install chromium. In a controlled deployment, install only the engine you use.
The examples below use Playwright’s synchronous API. They assume the target URL and injected script are trusted and that your process is allowed to reach them.
Load a JavaScript file by URL, wait for the app, and create a PDF
If the page already contains its own script tag, simply navigate to it. If it does not, add the remote file with page.add_script_tag(url=...). The call resolves when the script’s onload fires or its content has been injected; that does not prove that the application’s asynchronous rendering has finished.
from playwright.sync_api import sync_playwright
REPORT_URL = "https://example.test/report"
SCRIPT_URL = "https://example.test/app.js"
OUTPUT = "report.pdf"
with sync_playwright() as p:
browser = p.chromium.launch()
page = browser.new_page()
page.goto(REPORT_URL, wait_until="domcontentloaded")
page.add_script_tag(url=SCRIPT_URL)
# Replace this with the real readiness signal used by your application.
page.wait_for_function("window.reportReady === true")
# Optional: use screen CSS rather than Playwright's default print CSS.
# page.emulate_media(media="screen")
page.pdf(path=OUTPUT, format="A4", print_background=True)
browser.close()window.reportReady is only an example. Your page might set a global after an API response, add a .report-complete element, or expose a different state value. Use the signal that represents completed application work rather than copying the example flag.
Wait for a DOM element instead of a global flag
page.wait_for_selector("[data-report-ready='true']", state="visible")
page.pdf(path="report.pdf", format="A4", print_background=True)A selector is useful when you control the page markup. It is safer than a fixed sleep because the wait ends when the relevant element exists. For a page with several asynchronous phases, wait for the last phase that affects the PDF, such as charts finishing or a table reaching its final row count.
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from playwright.sync_api import sync_playwright
with sync_playwright() as p:
browser = p.chromium.launch()
page = browser.new_page()
page.goto("https://example.test/report", wait_until="networkidle")
page.wait_for_selector("#final-report", state="visible")
page.pdf(path="report.pdf", format="A4", print_background=True)
browser.close()networkidle is only a navigation heuristic. Analytics, WebSockets and polling can keep a page active indefinitely, while an application can finish rendering before the network becomes idle. Prefer an application-specific readiness check when one is available.
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Control the PDF layout
Print media versus screen media
page.pdf() uses print media by default, so @media print rules apply. If the page was designed for the screen and you want those rules, select screen media before printing:
page.emulate_media(media="screen")
page.pdf(path="screen-layout.pdf", format="A4", print_background=True)Check the result for hidden navigation, changed colors, page-break rules and responsive breakpoints. Selecting screen media does not disable PDF generation; it changes which CSS media query is active.
Common PDF options
path: destination filename.format="A4"(or another supported paper format): standard page sizing.print_background=True: include CSS background colors and images.margin: provide top, right, bottom and left margins when the stylesheet needs explicit print spacing.landscape=True: rotate the page orientation.page_ranges: emit selected page ranges when the document is long.
Use CSS @page rules for repeatable headers, footers and page breaks, and test with the actual browser engine installed in production.
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Confirm that the script was requested
from playwright.sync_api import sync_playwright
with sync_playwright() as p:
browser = p.chromium.launch()
page = browser.new_page()
page.on("console", lambda msg: print("console:", msg.type, msg.text))
page.on("pageerror", lambda exc: print("page error:", exc))
page.goto("https://example.test/report")
page.add_script_tag(url="https://example.test/app.js")
page.wait_for_selector("#final-report")
page.pdf(path="report.pdf")
browser.close()Console and page-error listeners expose syntax errors, blocked requests and runtime exceptions that otherwise look like an empty PDF.
Account for cross-origin and CSP behavior
The remote URL must be reachable from the browser process. Authentication, DNS, TLS, proxy settings and a site’s content-security policy can all prevent loading. If the application needs cookies or authorization headers, create a browser context with the required state before navigation. Do not weaken security policies globally just to make an injection work; fix the deployment or serve a trusted script from an allowed origin.
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Wait for data, fonts and images
JavaScript may finish inserting markup while charts, web fonts or images are still loading. Wait for the application’s final state and, where appropriate, verify that important images are complete in page JavaScript. A fixed delay can mask slow environments and still be too short under load, so use it only as a bounded supplement to a real readiness condition.
Security and isolation considerations
A remote script is executable code in the browser page. Restrict script origins and avoid allowing users to supply arbitrary URLs unless that is a deliberate, isolated feature. Treat the HTML, CSS and URLs as untrusted input when they come from customers.
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WeasyPrint’s security guidance warns about long renderings, high CPU or memory use, slow network requests and local-file access through file:// URLs. Apply the same defensive mindset to browser automation:
- Run rendering in a restricted worker or container.
- Set operation timeouts and enforce an overall job deadline.
- Limit outbound network destinations and block access to internal services.
- Sanitize or reject untrusted HTML and CSS.
- Control filesystem permissions and output paths.
- Review Chromium isolation settings. Playwright exposes a
chromium_sandboxlaunch option whose default is false; configure isolation explicitly for your deployment rather than assuming it is enabled.
If you must use WeasyPrint for static documents, a custom fetcher can filter protocols and paths. Do not let a user-controlled document freely read local files or make unrestricted network requests.
Troubleshooting common failures
The PDF is blank or missing dynamic content
Cause: WeasyPrint rendered before JavaScript ran, or Playwright printed before the application became ready.
Fix: Move the workflow to Playwright, add the script with add_script_tag if needed, and wait for a real DOM or application readiness signal.
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add_script_tag fails or the script has no effect
Cause: DNS/TLS failure, authentication, a blocked origin, a script syntax error or a page that expects the script in a particular order.
Fix: Listen for console and page errors, verify the URL from the rendering host, load prerequisite scripts first, and inspect the browser’s network logs.
The script loads but data is still incomplete
Cause: The script’s onload event only indicates that the file arrived; subsequent API calls and rendering continue afterward.
Fix: Wait for the final selector, state flag or application event. Avoid relying solely on networkidle.
The PDF looks different from the browser
Cause: Print media is the default, backgrounds may be disabled, or the viewport differs from a user’s screen.
Fix: Try page.emulate_media(media="screen"), set an explicit viewport, enable print_background=True, and add print-specific CSS where the PDF should intentionally differ.
Rendering hangs or consumes excessive resources
Cause: Polling, WebSockets, an unreachable resource, a pathological document or unbounded JavaScript work.
Fix: Use explicit navigation and operation timeouts, wait on a bounded readiness condition, cancel jobs that exceed a deadline, and enforce worker CPU and memory limits.
PDF/A validation fails
Cause: The selected PDF/A profile has restrictions that ordinary browser PDFs do not satisfy, including JavaScript-related requirements.
Fix: Confirm the required profile first and choose a renderer and post-processing pipeline that explicitly supports it.
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Performance, cost and operational design
Launching a browser for every document is simple but expensive compared with reusing a controlled browser process. Reuse contexts carefully, clear authentication state between tenants, and close pages after each job. Keep navigation, readiness and PDF timeouts separate so you can identify whether a failure is network, application or printing related.
Cache immutable scripts and static assets where policy permits, but do not cache user-specific responses across customers. Record the target URL, browser version, media mode, viewport, readiness condition and elapsed phases for diagnostics. Test slow API responses and missing assets, not only the fast path.
Or skip the browser setup
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Implementation checklist
- Confirm whether the source is static or JavaScript-dependent.
- Use Playwright when JavaScript must execute; use WeasyPrint only when static rendering is sufficient.
- Inject the remote script before application initialization when the page does not already include it.
- Wait for an application-specific readiness condition.
- Choose print or screen media deliberately.
- Set explicit viewport, page size, margins and background behavior.
- Constrain network, filesystem, CPU, memory and runtime for untrusted input.
- Capture console, page-error and timeout diagnostics.
- Validate the output against any PDF/A or accessibility requirement.
Frequently Asked Questions
Can I make WeasyPrint execute a remote JavaScript file with a custom fetcher?
No. A custom fetcher controls resource retrieval; it does not add a JavaScript runtime to WeasyPrint. Render the page in Playwright or another browser engine first.
Does Playwright’s script-load completion mean the PDF is ready?
No. It only indicates that the script file loaded or was injected. Wait for the application’s own completed state, such as a final selector or readiness flag.
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Can browser-rendered PDFs be used for PDF/A?
Only after checking the exact PDF/A profile and validating the generated file. PDF/A requirements can restrict JavaScript-related content and other browser-PDF characteristics.
Quick Recap
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