weasyprint( markup, fOut, opts, on_error ) {
    // Save the markup to a temporary file
    const tempFile = fOut.replace(/\.pdf$/i, '.pdf.html');
    FS.writeFileSync(tempFile, markup, 'utf8');

    SPAWN('weasyprint', [tempFile, fOut], false, on_error, this);
  }
Generate a PDF from HTML using WeasyPrint's CLI interface.
  Spawns a child process with `weasyprint <source> <target>`. Weasy Print
  must be installed and path-accessible.
  TODO: If HTML generation has run, reuse that output