OpenHtmlToPdf CSS Limitations: Working with a CSS 2.1 Renderer

Developers using OpenHtmlToPdf encounter a common pattern: layouts that render correctly in browsers appear broken or completely different in generated PDFs. Elements positioned with Flexbox collapse into vertical stacks, Grid layouts disappear entirely, and CSS variables simply do not work. The root cause is architectural: OpenHtmlToPdf implements CSS 2.1 with limited CSS3 support, not the full CSS specification that modern browsers use.

The Problem

OpenHtmlToPdf is a Java-based HTML-to-PDF library built on Flying Saucer and Apache PDF-BOX. It uses a custom rendering engine rather than a browser engine like WebKit or Chromium. This custom engine implements CSS 2.1 and selected CSS3 features, but explicitly excludes modern layout systems.

The project documentation states this directly:

"Be aware that you can not throw modern HTML5+ at this engine and expect a great result. You must special craft the HTML document for this library and use its extended CSS features to get good results."

When HTML containing modern CSS is processed:

• display: flex is completely ignored
• display: grid has no effect
• CSS custom properties (--variable-name) are not parsed
• opacity produces warnings and is ignored
• Many CSS3 properties generate "unrecognized property" warnings

Error Messages and Symptoms

OpenHtmlToPdf logs warnings for unsupported CSS properties:

WARNING: flex is an unrecognized CSS property at line 2. Ignoring declaration.
WARNING: -webkit-border-radius is an unrecognized CSS property at line 0. Ignoring declaration.
WARNING: opacity is an unrecognized CSS property at line 0. Ignoring declaration.
WARNING: word-break is an unrecognized CSS property at line 13. Ignoring declaration.
WARNING: clip-path is an unrecognized CSS property at line 0. Ignoring declaration.

In production, this manifests as:

• Flex containers render as display: block, stacking items vertically
• Grid items appear in a single column regardless of grid-template-columns
• Spacing properties like gap have no effect
• Transparency using rgba() or opacity produces solid colors
• Modern visual effects like box-shadow may not render

/* This CSS works in browsers but fails silently in OpenHtmlToPdf */
.dashboard {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 20px;
}

.card {
    display: flex;
    justify-content: space-between;
    align-items: center;
    opacity: 0.9;
    background: rgba(255, 255, 255, 0.95);
}

The output shows cards stacked vertically with no spacing, solid white backgrounds, and misaligned content.

Who Is Affected

This limitation affects developers in several scenarios:

Java Applications: OpenHtmlToPdf is a JVM library, commonly used in Spring Boot, Quarkus, and other Java frameworks for server-side PDF generation.

.NET Applications: The NuGet package "OpenHtmlToPdf" exists but is actually a wrapper for wkhtmltopdf, not a port of the Java library. This naming similarity causes confusion, as developers may expect the same capabilities. The .NET package inherits wkhtmltopdf's limitations (frozen WebKit from 2012) rather than OpenHtmlToPdf's specific constraints.

Use Cases Impacted:

• Invoice and report generation using modern CSS
• Dashboard exports requiring multi-column layouts
• Any workflow using CSS written after 2012
• Templates designed for browser rendering that need PDF output

CSS Features Not Available:

Feature	Status in OpenHtmlToPdf
CSS Flexbox	Not supported
CSS Grid	Not supported
CSS Custom Properties	Not supported
opacity	Not supported
overflow: hidden	Not supported
object-fit / object-position	Not supported
word-break	Not supported
CSS Animations	Not supported
calc()	Limited/Not supported
filter	Not supported
clip-path	Not supported

Evidence from the Developer Community

OpenHtmlToPdf's CSS limitations are documented in GitHub issues spanning several years.

Timeline

Date	Event	Source
2016-08-16	CSS3 support question opened	GitHub Issue #36
2017-01-11	Flexbox support requested	GitHub Issue #69
2018-02-07	Opacity not supported confirmed	GitHub Issue #226
2018-05-09	overflow:hidden not supported	GitHub Issue #273
2020-08-14	display:flex still not supported	GitHub Issue #531
2021-09-15	word-break unrecognized	GitHub Issue #718
2025-01-15	Active issues continue	GitHub Issues

Community Reports

"flex is an unrecognized CSS property at line 2. Skipping declaration."
— GitHub Issue #69, 2017

"at present display:flex There are many styles used."
— GitHub Issue #531, 2020

"opacity is an unrecognized CSS property at line 0. Ignoring declaration."
— GitHub Issue #226, 2018

The project FAQ addresses this directly:

"No, it's not a web browser. Specifically, it does not run javascript or implement many modern standards such as flex and grid layout."

Root Cause Analysis

OpenHtmlToPdf's CSS limitations stem from its architectural heritage:

Flying Saucer Foundation: OpenHtmlToPdf is built on Flying Saucer, a CSS 2.1 renderer written in pure Java. Flying Saucer was never designed to implement the full CSS specification—its goal was to render well-formed XHTML with CSS 2.1 styling.

Custom Rendering Engine: Unlike browser-based solutions that leverage WebKit, Blink, or Gecko, OpenHtmlToPdf uses a custom layout engine. Building a complete CSS3 implementation would require implementing:

1. The Flexbox layout algorithm (CSS Flexible Box Layout Module)
2. The Grid layout algorithm (CSS Grid Layout Module)
3. CSS Custom Properties cascade and inheritance
4. The CSS Object Model for modern properties

Each of these is a substantial specification requiring significant engineering effort.

Performance vs. Features Trade-off: The project documentation notes the engine is "not as well-maintained or fully-featured, but it is much lighter, allowing it to run significantly faster than alternatives." This is a deliberate trade-off: OpenHtmlToPdf can generate simple PDFs quickly but cannot handle modern CSS.

Scope of the Project: OpenHtmlToPdf focuses on PDF-specific features like accessible PDF generation (PDF/UA, Section 508, WCAG 2.0), PDF/A compliance, and SVG/MathML support. Implementing browser-equivalent CSS rendering is outside its scope.

When OpenHtmlToPdf Works

OpenHtmlToPdf is appropriate for specific use cases:

Simple Documents: Text-heavy documents with basic formatting (headings, paragraphs, lists) render well.

Table-Based Layouts: HTML tables with CSS styling work correctly.

Pre-2012 CSS Patterns: Layouts using floats, inline-block, and absolute positioning function as expected.

Accessible PDFs: The library excels at generating PDF/UA compliant documents for accessibility requirements.

SVG Integration: SVG graphics render correctly, which some alternatives struggle with.

<!-- This works well in OpenHtmlToPdf -->
<table style="width: 100%; border-collapse: collapse;">
    <tr>
        <td style="width: 50%; padding: 10px; border: 1px solid #ccc;">Column 1</td>
        <td style="width: 50%; padding: 10px; border: 1px solid #ccc;">Column 2</td>
    </tr>
</table>

When OpenHtmlToPdf Fails

OpenHtmlToPdf is not suitable when:

Modern CSS Is Required: Any layout using Flexbox or Grid will break.

Existing Web Templates: Templates designed for browser rendering typically use modern CSS and will not render correctly.

Visual Effects: Transparency, shadows, filters, and animations are not supported.

Complex Responsive Designs: Media queries have limited support, and responsive patterns rely on modern CSS.

<!-- This fails in OpenHtmlToPdf -->
<div style="display: flex; justify-content: space-between; gap: 20px;">
    <div style="flex: 1; opacity: 0.9;">Card 1</div>
    <div style="flex: 1; opacity: 0.9;">Card 2</div>
    <div style="flex: 1; opacity: 0.9;">Card 3</div>
</div>

Attempted Workarounds

Workaround 1: Table-Based Layouts

Approach: Replace Flexbox and Grid with HTML tables.

<!-- Instead of Flexbox -->
<table style="width: 100%;">
    <tr>
        <td style="width: 33%; padding: 10px;">Card 1</td>
        <td style="width: 33%; padding: 10px;">Card 2</td>
        <td style="width: 33%; padding: 10px;">Card 3</td>
    </tr>
</table>

Limitations:

• Requires complete template restructuring
• Tables have semantic issues for non-tabular data
• Vertical centering and equal-height columns require additional hacks
• Complex nested layouts become difficult to maintain

Workaround 2: Float-Based Layouts

Approach: Use CSS floats with clearfix patterns.

<div style="overflow: hidden;">
    <div style="float: left; width: 33%;">Card 1</div>
    <div style="float: left; width: 33%;">Card 2</div>
    <div style="float: left; width: 33%;">Card 3</div>
</div>

Limitations:

• overflow: hidden is also not supported in OpenHtmlToPdf
• Floats near page breaks cause rendering issues
• The project documentation specifically warns: "Avoid floats near page breaks"

Workaround 3: Maintain Separate Templates

Approach: Create simplified templates specifically for PDF generation.

Limitations:

• Doubles template maintenance effort
• Visual inconsistency between web and PDF output
• Changes must be synchronized across templates

Workaround 4: Suppress Warning Logs

Approach: Configure logging to hide CSS warnings.

// Attempting to suppress warnings
java.util.logging.Logger.getLogger("com.openhtmltopdf.css-parse")
    .setLevel(java.util.logging.Level.SEVERE);

Limitations:

• This hides symptoms, not the underlying problem
• Layouts still render incorrectly
• May hide legitimate parsing errors

A Different Approach: IronPDF

For developers who need to generate PDFs from modern HTML and CSS without rewriting templates, IronPDF offers an alternative architecture. IronPDF embeds a Chromium rendering engine—the same engine powering Google Chrome—providing complete CSS support.

Why IronPDF Supports Modern CSS

The architectural difference is fundamental:

Aspect	OpenHtmlToPdf	IronPDF
Rendering Engine	Custom CSS 2.1 parser	Chromium (Current)
CSS Flexbox	Not supported	Full support
CSS Grid	Not supported	Full support
CSS Variables	Not supported	Full support
Opacity/RGBA	Not supported	Full support
JavaScript	Not supported	Full V8 engine
Platform	JVM (Java)	.NET (C#)

IronPDF's Chromium engine renders CSS exactly as Chrome does. The same HTML that works in a browser works identically in IronPDF.

Code Example

using IronPdf;

public class ModernLayoutPdfGenerator
{
    public void GenerateDashboardReport()
    {
        // IronPDF uses Chromium, supporting all modern CSS
        var renderer = new ChromePdfRenderer();

        string html = @"
<!DOCTYPE html>
<html>
<head>
    <style>
        :root {
            --primary-color: #3b82f6;
            --card-radius: 8px;
            --spacing: 16px;
        }

        body {
            font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
            margin: 0;
            padding: var(--spacing);
            background: #f3f4f6;
        }

        .header {
            display: flex;
            justify-content: space-between;
            align-items: center;
            padding: var(--spacing);
            background: var(--primary-color);
            color: white;
            border-radius: var(--card-radius);
            margin-bottom: var(--spacing);
        }

        .dashboard {
            display: grid;
            grid-template-columns: repeat(3, 1fr);
            gap: var(--spacing);
        }

        .card {
            background: white;
            border-radius: var(--card-radius);
            padding: var(--spacing);
            box-shadow: 0 1px 3px rgba(0, 0, 0, 0.1);
        }

        .card-header {
            display: flex;
            justify-content: space-between;
            align-items: center;
            margin-bottom: 12px;
            padding-bottom: 12px;
            border-bottom: 1px solid #e5e7eb;
        }

        .card-title {
            font-weight: 600;
            color: #374151;
        }

        .card-badge {
            background: rgba(59, 130, 246, 0.1);
            color: var(--primary-color);
            padding: 4px 8px;
            border-radius: 4px;
            font-size: 12px;
        }

        .card-value {
            font-size: 2rem;
            font-weight: 700;
            color: #111827;
        }

        .footer {
            display: flex;
            justify-content: center;
            margin-top: var(--spacing);
            padding-top: var(--spacing);
            color: #6b7280;
            font-size: 14px;
        }

        @media print {
            body { background: white; }
            .card { box-shadow: none; border: 1px solid #e5e7eb; }
        }
    </style>
</head>
<body>
    <div class='header'>
        <h1 style='margin: 0; font-size: 1.5rem;'>Q4 Performance Report</h1>
        <span>Generated: " + DateTime.Now.ToString("yyyy-MM-dd") + @"</span>
    </div>

    <div class='dashboard'>
        <div class='card'>
            <div class='card-header'>
                <span class='card-title'>Revenue</span>
                <span class='card-badge'>+12.5%</span>
            </div>
            <div class='card-value'>$2.4M</div>
        </div>

        <div class='card'>
            <div class='card-header'>
                <span class='card-title'>Active Users</span>
                <span class='card-badge'>+8.3%</span>
            </div>
            <div class='card-value'>48,291</div>
        </div>

        <div class='card'>
            <div class='card-header'>
                <span class='card-title'>Conversion Rate</span>
                <span class='card-badge'>+2.1%</span>
            </div>
            <div class='card-value'>4.8%</div>
        </div>
    </div>

    <div class='footer'>
        <span>Data refreshed automatically</span>
    </div>
</body>
</html>";

        using var pdf = renderer.RenderHtmlAsPdf(html);
        pdf.SaveAs("dashboard-report.pdf");
    }
}

Key points about this code:

• CSS Variables work with var() function
• CSS Grid creates the three-column layout
• Flexbox handles header and card internal alignment
• rgba() transparency renders correctly
• box-shadow produces proper visual effects
• @media print query applies print-specific styles

API Reference

For detailed documentation:

• ChromePdfRenderer - The primary rendering class
• HTML to PDF Tutorial - Complete guide with examples
• CSS Support in IronPDF - CSS feature documentation

Migration Considerations

Platform Difference

OpenHtmlToPdf is a Java library; IronPDF is a .NET library. Migration requires a platform shift for Java applications. For .NET developers already using the similarly-named "OpenHtmlToPdf" NuGet package (which wraps wkhtmltopdf), IronPDF is a direct replacement.

Licensing

• OpenHtmlToPdf: Open source (LGPL-compatible via PDF-BOX)
• IronPDF: Commercial software with per-developer licensing
• IronPDF Licensing Information

What You Gain

• Complete CSS3 support (Flexbox, Grid, Variables)
• Same templates for web and PDF output
• JavaScript execution for dynamic content
• Modern rendering engine with security updates
• Active development and support

What to Consider

• Commercial licensing cost
• Platform change if migrating from Java
• Larger deployment footprint (embedded Chromium)
• Higher memory usage during rendering

Conclusion

OpenHtmlToPdf implements a deliberate trade-off: fast, lightweight PDF generation in exchange for limited CSS support. For simple documents with table-based layouts, it functions well. For modern CSS layouts using Flexbox, Grid, or CSS custom properties, the library is not suitable and workarounds are inadequate. Developers requiring browser-equivalent CSS rendering need a solution built on a current rendering engine. IronPDF's embedded Chromium engine renders modern CSS without requiring template modifications.

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Jacob Mellor has spent 25+ years building developer tools, including IronPDF.

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References

1. OpenHtmlToPdf GitHub Repository{:rel="nofollow"} - Official community fork
2. danfickle/openhtmltopdf{:rel="nofollow"} - Original repository
3. Flexbox Support Issue #69{:rel="nofollow"} - Flexbox feature request
4. CSS3 Support Issue #36{:rel="nofollow"} - CSS3 support discussion
5. Opacity Not Supported Issue #226{:rel="nofollow"} - Opacity limitation
6. display:flex Issue #531{:rel="nofollow"} - Flexbox not working
7. overflow:hidden Issue #273{:rel="nofollow"} - Overflow property not supported
8. Flying Saucer Project{:rel="nofollow"} - Underlying CSS 2.1 renderer

For the latest IronPDF documentation and tutorials, visit ironpdf.com.