Cross-engine HTML→PDF regression - generate the same source HTML through Chromium `page.pdf()`, WeasyPrint (Python, BSD), and wkhtmltopdf, then compare per-engine outputs page-by-page. Catches engine-specific font embedding bugs, page-break drift, @page rule support gaps. Pair with pdf-snapshot-tester for the per-engine pixel diff. Use when a project is swapping or upgrading its PDF engine, or when one shared HTML template is rendered by more than one engine and the outputs must stay equivalent.