“wkhtmltopdf-wasm” is not clearly identified as an official browser build of wkhtmltopdf. The closest documented browser-WebAssembly option is the opt-in adapter in the separate gowkhtmltopdf project. It accepts inline HTML through a JavaScript bridge and returns PDF, PNG, or JPEG bytes. The instructions below apply to that adapter, not to the upstream command-line program.
First, identify which project you mean
The name matters because the browser workflow is not the same as running the original wkhtmltopdf executable. The upstream project describes wkhtmltopdf and wkhtmltoimage as LGPLv3 command-line tools built around Qt WebKit; its documented route is to download or build a binary, prepare HTML, and run the tool on it (upstream wkhtmltopdf).
The browser-WASM workflow documented here belongs to gowkhtmltopdf, a separate Go project. Its opt-in browser adapter runs a Go document pipeline in a Web Worker and returns bytes for PDF, PNG, or JPEG. A separately named project, wasm-pdf, also creates PDFs in the browser using JavaScript and WebAssembly, but its documented input is a JSON document model, not the wkhtmltopdf command-line interface. Do not treat these projects as interchangeable.
What the gowkhtmltopdf browser adapter can and cannot do
Its useful fit
Use the adapter when you can provide the document as inline HTML and want a browser-side conversion to PDF or an image. The bridge takes a JSON request and can report progress through an optional callback. For a PDF request, the documented options include page size and orientation. Image output supports width, height, padding, and JPEG quality under mode-specific rules.
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Important rendering limits
The adapter does not execute JavaScript embedded in the document. It does not read browser files, fetch arbitrary remote resources, or claim to reproduce a full browser renderer. The documentation says local-file access, system-font discovery, and network schemes are disabled; remote CSS, images, and fonts are not promised. Inline resources such as data URLs can be embedded in the HTML.
That makes it a poor fit for printing a live site whose layout depends on scripts or remotely hosted assets. Before adopting it, test the actual markup, styles, fonts, images, page breaks, and pagination your output needs. For a public URL that must render as a visitor sees it, a screenshot/PDF service or a full browser automation workflow may be more suitable.
Build the browser WASM files
The current gowkhtmltopdf getting-started documentation specifies Go 1.26 or a compatible toolchain; check the project’s release documentation because toolchain requirements can change (gowkhtmltopdf project documentation). The following commands are run from the repository root:
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Build the WASM adapter and stage its runtime and supporting files:
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Build the frontend:
npm --prefix frontend run build
The build targets GOOS=js GOARCH=wasm. The project’s make target stages gowkhtmltopdf.wasm, the active Go toolchain’s wasm_exec.js, and sample/manifest files in the frontend’s public WASM directory. Use the generated runtime and binary together; they are a matched output of the build, not files to substitute casually from another Go version.
Start the Go runtime and request a PDF
The JavaScript side starts the Go runtime through wasm_exec.js. Once the WASM program has initialized, it registers globalThis.gowkhtmltopdfWASM. The project bridge’s request shape includes inline HTML, a mode, and PDF options. A minimal page-side integration follows the documented initialization and request pattern; use the actual generated asset paths from the build output in your application:
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// Load wasm_exec.js before this code, using the file staged by `make wasm`.
const go = new Go();
const wasmResponse = await fetch('/wasm/gowkhtmltopdf.wasm');
const wasmBytes = await wasmResponse.arrayBuffer();
const { instance } = await WebAssembly.instantiate(wasmBytes, go.importObject);
// Go.run starts the runtime. The bridge is registered during startup.
const runtimePromise = go.run(instance);
while (!globalThis.gowkhtmltopdfWASM) {
await new Promise(resolve => setTimeout(resolve, 0));
}
const result = await globalThis.gowkhtmltopdfWASM({
html: '<!doctype html><html><body><h1>Invoice</h1><p>Amount due: $42</p></body></html>',
mode: 'pdf',
pageSize: 'A4',
orientation: 'portrait'
}, progress => {
console.log('Conversion progress:', progress);
});
if (!result.ok) {
throw new Error(`${result.code}: ${result.message}`);
}
const pdfBlob = new Blob([result.bytes], { type: result.mime });
const link = document.createElement('a');
link.href = URL.createObjectURL(pdfBlob);
link.download = 'invoice.pdf';
link.click();
URL.revokeObjectURL(link.href);This illustrates the integration pattern rather than guaranteeing a particular asset URL or application bundler configuration. Follow the current project example for exact request field names and runtime loading in the release you build. On success, the response includes ok, mode, mime, a version value, dimensions where relevant, and a Uint8Array of output bytes. On failure it returns an error code and message. Check ok before trying to download the bytes.
Use a worker for an interactive application
The project frontend uses a worker for live preview: it starts the runtime once, sends requests with IDs, transfers result bytes, reports progress, and rejects pending requests if the worker is cancelled. A worker keeps conversion work away from the main UI thread, but it does not remove the adapter’s input, output, or time limits. If you implement your own worker, associate each reply with its request ID and reject outstanding promises on cancellation or worker failure so the interface cannot hang waiting for a result.
Choose output mode and control document size
- PDF: provide inline HTML and, when needed, page size and orientation. Validate page breaks and content flow with representative documents rather than assuming complex browser print behavior.
- PNG or JPEG: use the image mode’s supported width, height, and padding settings; JPEG mode also accepts a quality value under the documented rules. Check the resulting dimensions before displaying or storing the bytes.
- Embedded assets: put required images or other supported resources directly into the HTML, such as data URLs. Do not rely on arbitrary remote fetches, local filesystem paths, or automatic system-font discovery.
The gowkhtmltopdf browser-WASM documentation describes an HTML input limit of 4 MiB, an output limit of 32 MiB, image dimensions capped at 4096 pixels per axis, and a 60-second context for one worker conversion. These are implementation limits for the documented adapter, not limits imposed universally by WebAssembly. Verify them against the release you deploy, especially if documents include large embedded images or generate long PDFs.
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Security, reliability, and performance considerations
Keep untrusted HTML contained
The documented browser adapter disables network schemes and local-file access, which limits the resource-loading behavior described above. Still treat HTML as untrusted input: sanitize content according to your application’s needs, constrain who can submit it, and handle conversion errors. WebAssembly is not an automatic security boundary. The W3C WebAssembly Web API says its threat model is essentially the same as JavaScript’s and that the Wasm format itself provides no integrity or privacy protection; delivery protections such as HTTPS and application policy remain important (W3C WebAssembly Web API).
Plan for memory and cancellation
Converting a document requires memory for the runtime, input, layout work, and output. Large embedded assets and high-resolution images can raise memory use; remain comfortably below the documented input/output and image-size limits. For interactive use, run conversions in a worker and expose cancellation. The project’s frontend behavior rejects pending worker requests when the worker is cancelled; mirror that behavior if you build a custom message protocol.
Do not confuse native URL conversion with browser conversion
The native Go API can be relevant when you need local-file input, URL input, custom font paths, or native network policy. But accepting arbitrary user-provided URLs in a server-side converter can create server-side request forgery (SSRF) risk. The project’s getting-started guide recommends host allowlists and network isolation for such integrations. That warning concerns native URL-fetching use; it does not describe the browser adapter, which disables network schemes.
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Troubleshooting common failures
gowkhtmltopdfWASMis undefined: ensurewasm_exec.jsloads before the application code, use the matching binary staged bymake wasm, and wait for the Go runtime to initialize before making a request.- The WASM file fails to load or instantiate: verify the public asset path matches your build output, the response is the actual binary rather than an HTML fallback page, and the runtime and binary came from the same build/toolchain.
- Fonts, CSS, or images are missing: the adapter does not promise arbitrary remote or local resource loading or system-font discovery. Embed supported assets in the HTML and test the exact document. If it depends on a live site’s scripts or network resources, use a renderer designed to load that site.
- Output is blank or incomplete: remove assumptions about document JavaScript execution, check that the HTML contains its final content before conversion, and reduce oversized embedded resources. The adapter does not run document scripts.
- Conversion returns an error code: inspect both the returned code and message; confirm the request’s mode-specific options and keep input/output and image dimensions within the documented adapter limits.
- The page becomes unresponsive: move conversion to a worker as in the project frontend, report progress, and provide cancellation. A worker protects responsiveness but does not make an over-limit conversion succeed.
- A native server conversion can reach an unintended host: do not pass arbitrary URLs through unchecked. Apply strict host allowlists and network isolation, as recommended by the project guide.
When a different PDF approach is a better match
If you need to construct a PDF from application data rather than render existing HTML, PDFKit is a distinct JavaScript option documented for Node and browsers. Its project describes vector drawing, text, embedded fonts and images, tables, annotations, forms, encryption, and accessibility support (PDFKit repository). That is programmatic document generation, not an equivalent HTML-page renderer.
If the requirement is to capture a live public website with its browser-loaded content, first compare whether the tool actually runs page JavaScript, fetches remote CSS and assets, and handles the print layout you need. Do not choose the gowkhtmltopdf browser adapter on the assumption that “WASM” means full browser rendering.
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Does the browser adapter run JavaScript inside the HTML document?
No. Its documentation explicitly says document JavaScript does not run.
Can I use this to convert an arbitrary web page URL in the browser?
Not with the documented browser adapter: it accepts inline HTML and disables network schemes. Consider a browser-rendering service for a live URL.
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