You cannot convert HTML to WebP with cURL alone. cURL sends the request; a browser-backed renderer must turn HTML or a webpage into pixels, and the renderer or a separate encoder must produce WebP. For a hosted workflow, send HTML or a public URL to a rendering API that supports WebP. For a local workflow, capture a browser screenshot and convert the resulting image.
What cURL does—and what it does not do
cURL is an HTTP client. It can submit HTML, a webpage URL, and output settings to a service, then save the response. It does not lay out a page, run its JavaScript, or encode image pixels. Those tasks belong to a browser engine and, when necessary, an image encoder.
Chrome’s documentation makes this distinction: cURL can fetch HTML source, while Chrome parses HTML, executes scripts that can change the page, and builds a DOM. A source file is therefore not a screenshot: it may omit styles, images, fonts, and content inserted after scripts run. See Chrome Headless command-line reference.
The right route depends on your input and where you want rendering to happen: a hosted API for HTML or a public URL, a self-hosted Chromium service, or Chrome locally followed by WebP encoding.
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Hosted route: send HTML or a webpage to a renderer
Render supplied HTML and CSS with HCTI
HCTI documents an authenticated cURL POST to https://hcti.io/v1/image that accepts HTML and CSS and requests WebP using format=webp. Its documentation describes a managed Chromium environment and a returned hosted WebP URL. This is the vendor-documented pattern, not an independently run test; obtain credentials and check the provider’s current requirements before using it. See HCTI documentation.
curl --fail-with-body --request POST 'https://hcti.io/v1/image'
--user "$HCTI_API_ID:$HCTI_API_KEY"
--data-urlencode 'html=<div class="card"><h1>Hello, world!</h1></div>'
--data-urlencode 'css=.card { width: 480px; padding: 40px; }'
--data-urlencode 'format=webp'Set HCTI_API_ID and HCTI_API_KEY in your shell or secrets manager rather than embedding real credentials in a script or source repository. --data-urlencode safely encodes the HTML and CSS as form fields, including spaces and punctuation. --fail-with-body makes HTTP errors visible to scripts while retaining the response body for diagnosis; it requires a recent cURL version that supports the option.
The documented result is a hosted image URL, not necessarily the image bytes written directly to a local file. Inspect the response and retrieve the returned image URL separately if you need a local file. Confirm the response format and content type rather than assuming a successful HTTP status means the payload is an image.
Render a public webpage URL
HCTI also documents sending a public webpage URL, viewport dimensions, and format=webp to the same endpoint. Use the provider’s current field names and authentication details for URL captures. This route is appropriate when the target is publicly accessible to the provider’s browser; a page behind your local network or login may not be reachable without a supported authentication mechanism.
A URL screenshot captures a rendered view, not the original document’s source. Choose a viewport that fits the content you want, and account for dynamic content: a page may need time for scripts, images, or fonts to load. Do not assume that every renderer waits for the same page-ready condition.
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Other ways to request a WebP screenshot
ScreenshotNeo: a URL-to-WebP API with clean-shot handling
ScreenshotNeo accepts a URL in a GET request and can return WebP, PNG, JPEG, or PDF. Its documented differentiator is that it accepts cookie/consent banners like a visitor and removes more than 60 known consent platforms, newsletter popups, and chat widgets before capture; those steps can be turned off. Bot checks and CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and response headers identify the page verdict and billing status.
For a public webpage screenshot, request WebP with format=webp:
curl -G "https://api.screenshotneo.com/v1/shot"
-d access_key=YOUR_API_KEY
--data-urlencode url=https://stripe.com
-d format=webp
-o shot.webpSee the ScreenshotNeo API documentation for authentication and request options. This example captures a URL, not an HTML string supplied in the request. ScreenshotNeo also provides an MCP server with take_screenshot, get_page_info, and capture_pdf tools for Claude, Cursor, and other MCP clients.
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Pricing is $0 for 1,000 shots per month with no card, then $5 for 3,000, $15 for 15,000, $39 for 60,000, $99 for 250,000, or $249 for 1,000,000; yearly billing gives two months free. For the broader workflow, the key distinction is that the GET call can handle URL capture and return WebP directly, while the HCTI example accepts provided HTML/CSS as well as a webpage URL.
Headless-Render-API for URL captures
Headless-Render-API documents cURL controls for capturing a public webpage and selecting WebP with an HTTP header. Its request options include capture settings. This can suit scripts that need header-based controls, but check its documentation for the exact current header names, authentication, and response handling: Headless-Render-API documentation.
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Gotenberg for a self-hosted Chromium service
Gotenberg documents a Chromium screenshot endpoint that takes an HTML file and optional assets and supports WebP output. It is a self-hosted route: you operate or arrange the service rather than relying on a hosted screenshot API. Its documentation warns that pages relying on JavaScript may be captured before rendering has finished, so make page readiness part of your implementation. See Gotenberg HTML-to-screenshot documentation.
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Local route: capture with Chrome, then encode WebP
Chrome Headless can render a page and save a screenshot. Google’s documented command-line screenshot example produces PNG; use Google’s cwebp utility to encode that image as WebP. The separate steps make the distinction clear: Chrome renders pixels, then cwebp converts an image file. See the Chrome Headless reference and cwebp documentation.
-
Install Chrome or Chromium and make sure the
chromeexecutable is on your PATH. Install Google’s WebP tools, includingcwebp, using the instructions for your operating system. -
Capture the page. Replace the URL with the page you need:
chrome --headless --screenshot=page.png https://example.com -
Encode the screenshot:
cwebp page.png -o page.webp -
Check that the output exists and is a WebP image. Keep the PNG if you need a lossless source for another conversion or want to compare the output visually.
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Chrome’s screenshot command-line example is a basic capture, not a universal recipe for full-page output, authentication, custom viewport, or waiting for a particular application state. Those needs may call for appropriate browser flags or an automation framework; confirm supported flags for the Chrome version installed on your machine. If a page is populated asynchronously, a command that captures too soon may produce an incomplete image.
Choosing a route
| Route | Input and output documented | Where rendering happens | Important consideration |
|---|---|---|---|
| ScreenshotNeo | Public URL; WebP, PNG, JPEG, or PDF | Hosted API | Consent and popup cleanup options; billed only for clean shots. Use the API docs for exact settings. |
| HCTI | HTML/CSS or public URL; WebP request and viewport controls | Managed Chromium service | Authenticated request; documentation describes a returned hosted WebP URL. |
| Headless-Render-API | Public URL; WebP selected through an HTTP header | Hosted API | Consult its docs for current header and response details. |
| Gotenberg | HTML file and optional assets; Chromium screenshot in WebP | Self-hosted service | JavaScript-dependent pages can be captured before they finish rendering. |
| Chrome plus cwebp | Rendered screenshot file, then WebP encoding | Your local machine | Separate browser and encoder setup; Chrome’s documented screenshot example saves PNG. |
Choose based on whether your source is HTML/CSS or a URL, whether you want a managed provider or local control, how the page’s JavaScript and load timing are handled, which viewport or crop controls are available, whether WebP is returned directly, and how authentication and response files are managed. The cited documentation does not establish a comparative speed, image quality, price, reliability, or usage-limit ranking across HCTI, Headless-Render-API, Gotenberg, and local Chrome.
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For a public URL, ScreenshotNeo can return a WebP capture in one request. Cookie banners, popups, and chat widgets are removed before the shot; bot checks, blank pages, and failed loads are never billed. An MCP server lets AI agents take screenshots. The Free plan includes 1,000 screenshots a month with no card, and paid plans start at $5 for 3,000.
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpSee the API documentation for request options. Sign up free for 1,000 screenshots a month with no card.
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The output is HTML or JSON, not an image
A renderer may return a JSON response containing a hosted image URL, or an error body, rather than image bytes. Inspect the response and content type. Follow the returned image URL if the provider documents that response shape; save bytes with -o only when the endpoint returns the image itself.
The page is blank or missing content
Check that the URL is publicly reachable by the rendering service and that the page does not require an unprovided login or network access. If JavaScript fills the page, allow for its load timing or configure a documented readiness wait. Gotenberg specifically warns that JavaScript-driven content may not be ready when its screenshot is taken.
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cURL reports an authentication or request error
Confirm the API credentials, endpoint, required fields, and the provider’s current authentication method. In the HCTI example, credentials are supplied through --user using environment variables. Avoid placing secrets in shell history or logs. A failed HTTP status with a body can provide a useful clue when using a cURL version that supports --fail-with-body.
The file has the wrong format
For an API, check that the WebP format parameter or header matches that service’s documented syntax and inspect the response content type. In the local route, Chrome’s screenshot example creates PNG; run cwebp on that file to produce WebP. Renaming a PNG file to end in .webp does not convert its encoding.
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The WebP is cropped or the layout differs
Set the viewport or capture dimensions supported by the renderer and verify whether it captures a viewport, full page, or a selected element. Responsive pages reflow at different widths, so match the target viewport before comparing results. If a provider offers a full-page or selector capture option, use its documentation to confirm the exact setting.
WebP support and fallback
Chrome developer documentation says current major browsers support WebP, while advising a PNG or JPEG fallback when older browser support is required. If the image will be served to an audience with unknown browser versions, provide a fallback rather than assuming every client can display WebP. See WebP image guidance.
Frequently Asked Questions
Can cURL convert an HTML file directly into a WebP file?
No. cURL sends requests and saves responses; a browser renderer must first turn HTML into pixels, and WebP encoding must happen in the renderer or a separate encoder.
Does a .webp filename make a PNG into WebP?
No. The file must be encoded as WebP by a renderer that returns WebP or by an encoder such as cwebp.
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