Openpdf-html: Improve HTML5 and modern CSS support

[andreasrosdal]

https://github.com/LibrePDF/OpenPDF/tree/master/openpdf-html

💡 Ideas to Improve openpdf-html

• Improve HTML5 and CSS support!

• Consider using Jsoup for HTML parsing. https://jsoup.org/

1. Improve @media query handling

   • ✅ Partial support exists — expand support for more media types (e.g., screen, all, min-width, max-width)
   • 📈 Benefit: Enables more responsive PDF rendering based on CSS print rules.

2. Add support for display: flex (basic Flexbox)

   • ⛔ Currently unsupported
   • 📈 Benefit: Allows rendering of modern layouts commonly used in HTML/CSS today.

3. Enhance <table> rendering

   • 🛠 Improve support for thead, tfoot, colspan, rowspan, and automatic header repetition across pages.
   • 📈 Benefit: Enables more professional and structured reports.

4. Implement position: fixed for headers/footers

   • ⛔ Not currently rendered as fixed
   • 📈 Benefit: Allows sticky headers or footers for printed documents, great for page numbers and branding.

5. Improve font fallback and international character support

   • 🛠 Improve behavior when specified fonts are missing; better default to system or bundled fonts.
   • 📈 Benefit: Better support for multilingual PDFs and consistent rendering.

6. Add support for <svg> elements

   • ⛔ Not supported
   • 📈 Benefit: Enables vector graphics, icons, and charts to be rendered natively.

7. Improve image rendering (e.g. <picture>, srcset)

   • 🛠 Support HTML <picture> tag and srcset attributes to choose optimal image resolution.
   • 📈 Benefit: Better image quality and optimization in PDF output.

8. Support @font-face for embedded custom fonts

   • ⛔ Not yet supported
   • 📈 Benefit: Allows authors to use brand fonts directly from CSS in the final PDF.

9. Render CSS box-shadow and text-shadow

   • ⛔ Shadows are currently ignored
   • 📈 Benefit: Improves fidelity of modern UI and design-heavy HTML.

10. Expose a debug/render tree output for PDF generation

    • 🧪 Provide a debug mode that outputs the parsed render tree or layout model.
    • 📈 Benefit: Makes debugging layout issues much easier for developers.

See: https://github.com/flyingsaucerproject/flyingsaucer/issues