Rend

A high-fidelity HTML/CSS rendering engine for .NET. Converts HTML to PDF or images (PNG, JPEG, WebP) with pixel-perfect accuracy.

Status: Active development. Focused on Chrome-level rendering accuracy. The public API will evolve — expect breaking changes.

Quick Start

// HTML to PDF
byte[] pdf = Render.ToPdf("<h1>Hello, World!</h1>");
File.WriteAllBytes("output.pdf", pdf);

// HTML to image
byte[] png = Render.ToImage("<h1>Hello, World!</h1>");
File.WriteAllBytes("output.png", png);

With Options

var options = new RenderOptions
{
    PageSize = Rend.Core.Values.PageSize.Letter,
    MarginTop = 48f,
    MarginRight = 48f,
    MarginBottom = 48f,
    MarginLeft = 48f,
    Title = "Invoice #1234",
    Author = "Acme Corp",
    ImageFormat = "png",
    Dpi = 150f,
};

byte[] pdf = Render.ToPdf(html, options);

Async

byte[] pdf = await Render.ToPdfAsync(html, options, cancellationToken);

// Or write directly to a stream
await Render.ToPdfAsync(html, outputStream, options, cancellationToken);

Headers and Footers

var options = new RenderOptions
{
    HeaderHtml = "<div style='font-size:10px;text-align:center'>Page {pageNumber} of {totalPages}</div>",
    FooterHtml = "<div style='font-size:9px;text-align:right'>{date}</div>",
};

Progress Reporting

var progress = new Progress<RenderProgress>(p =>
    Console.WriteLine($"[{p.Percentage}%] {p.Stage}: {p.Description}"));

var options = new RenderOptions { Progress = progress };

Custom Fonts

var fonts = new FontCollection();
fonts.RegisterFromResolver(new SystemFontResolver());
fonts.RegisterFromResolver(new DirectoryFontResolver("/path/to/fonts"));

var options = new RenderOptions { FontProvider = fonts };

Features

Layout

• Block, inline, and inline-block formatting contexts
• Flexbox (all axes, wrapping, alignment, gap, order, flex-grow/shrink)
• CSS Grid (auto-placement, fr/minmax/repeat, auto-fill/fit, named lines/areas, subgrid)
• Table layout (fixed and auto, rowspan/colspan, border-collapse)
• Positioned elements (relative, absolute, fixed, sticky)
• Floats and clear
• Multi-column layout
• Pagination with page breaks, orphans, and widows

CSS

• Full cascade, specificity, and inheritance
• calc(), var(), custom properties
• @media, @font-face, @import, @page, @supports
• CSS Color Level 4
• Shorthand expansion

Rendering

• Backgrounds (solid, gradients, images, multiple)
• Borders (all styles, border-radius, border-image)
• Box shadows, text shadows
• CSS transforms, opacity, filters, clip-path
• Linear, radial, and conic gradients
• SVG rendering
• text-overflow: ellipsis

Text

• HarfBuzz text shaping
• Unicode line/word breaking (UAX #14, #29)
• Bidirectional text (UAX #9)
• White-space handling, text-transform, letter/word-spacing
• Font fallback chains
• WOFF/WOFF2 support
• System font discovery

PDF Output

• PDF 1.7 with font subsetting (TrueType, CFF/OpenType)
• Bookmarks, clickable links, document metadata
• AcroForms (text fields, checkboxes, dropdowns)
• Encryption (RC4-128, AES-128)
• ICC color profiles, XMP metadata

Image Output

• PNG, JPEG, WebP via SkiaSharp
• Configurable DPI

Architecture

HTML string
     |
     v
[ HTML Parser ]        WHATWG HTML5 spec
     |
     v
[ CSS Parser ]         CSS Syntax Level 3
     |
     v
[ Style Resolution ]   Cascade, specificity, inheritance
     |
     v
[ Layout Engine ]      Block, inline, flex, grid, table, floats, positioning
     |
     v
[ Painter ]            Abstract drawing commands
     |
     +---------+---------+
     |                   |
     v                   v
[ PDF Target ]    [ Skia Target ]
     |                   |
     v                   v
  PDF bytes        Image bytes

Projects

Project	Description
Rend.Core	Shared types — geometry, color, units
Rend.Html	HTML5 parser, DOM, selector engine
Rend.Css	CSS3 parser, cascade, style resolution
Rend.Pdf	Standalone PDF 1.7 writer (no HTML/CSS dependency)
Rend	Layout, rendering, text shaping, fonts, output bridges, orchestrator

All projects target netstandard2.0 (.NET Framework 4.6.1+, .NET Core 2.0+, .NET 5–9+).

Standalone PDF Writer

Rend.Pdf is independently usable — no HTML/CSS dependency:

using Rend.Pdf;

var doc = new PdfDocument();
var page = doc.AddPage(595.28f, 841.89f);
var font = doc.AddFont(fontData);

var cs = page.ContentStream;
cs.BeginText();
cs.SetFont(font, 12f);
cs.SetTextPosition(72f, 750f);
cs.ShowText(font.Encode("Hello from Rend.Pdf!"));
cs.EndText();

using var stream = File.Create("output.pdf");
doc.Save(stream);

Building

Requires .NET 8 SDK.

dotnet build Rend.sln
dotnet test Rend.sln

License

MIT