HTML email not rendering in flutter

Summary

HTML emails fail to render in a Flutter email client app while plain text emails display correctly. The issue stems from using the flutter_html package for rendering, which relies on an outdated HTML engine (flutter-renderer) that does not support modern CSS and interactive features required by many HTML emails. The Html widget silently fails (often showing a blank space) when encountering unsupported tags or styles, rather than raising an error.

Root Cause

The primary cause is the limitations of the flutter_html package’s Html widget. It uses a WebViews-based or custom rendering engine that lacks full compliance with web standards. Email clients (Gmail, Outlook, etc.) generate complex HTML/CSS that utilizes features like:

External Resources: <img src="..."> or <link href="..."> which require explicit permission to load (often blocked by default).
Complex CSS: Flexbox, Grid, or specific positioning properties that flutter_html does not support.
Interactive Elements: JavaScript or complex form events, which are stripped or ignored.
Malformed HTML: Emails often contain invalid HTML structures that strict parsers reject.

Why This Happens in Real Systems

In production environments, third-party packages often lag behind evolving web standards due to:

Maintenance Overhead: Keeping a universal HTML renderer compatible with the entire CSS/HTML spec is incredibly difficult.
Scope Creep: flutter_html was designed for simple static content (like blog posts), not full email clients which require “mini-browsers” capabilities.
Silent Failures: Unlike native crashes, rendering engines often fail silently on unsupported tags (rendering a white screen) to avoid breaking the UI layout, making debugging difficult.

Real-World Impact

Poor User Experience: Users perceive the app as broken because expected content (images, formatted text) is missing.
Data Loss Risk: Critical information contained within HTML-specific formatting (tables, lists) may be invisible to the user.
Developer Velocity: Junior developers spend days debugging “working code” that simply hits a package limitation.
App Store Rejections: If the email client cannot display attachments or formatted content correctly, it fails the “use case” test during review.

Example or Code (if necessary and relevant)

The provided code triggers the error because _sanitizeHtml only parses the string but doesn’t strip unsupported tags or fix the root cause. The Html widget receives complex HTML it cannot process.

// This code snippet demonstrates the problematic flow in the original implementation.
// It attempts to render complex HTML using flutter_html, which often fails silently.

import 'package:flutter/material.dart';
import 'package:flutter_html/flutter_html.dart';
import 'package:html/parser.dart' as html_parser;

// A simplified version of the email rendering logic
class EmailRenderer extends StatelessWidget {
  final String htmlContent;

  const EmailRenderer({super.key, required this.htmlContent});

  String _sanitizeHtml(String content) {
    try {
      // This parsing step is insufficient for fixing rendering issues
      final document = html_parser.parse(content);
      return document.outerHtml;
    } catch (e) {
      return content; // Fallback to raw HTML
    }
  }

  @override
  Widget build(BuildContext context) {
    return SingleChildScrollView(
      child: Html(
        data: _sanitizeHtml(htmlContent),
        style: {
          // Unsupported CSS will be ignored or break layout
          'body': Style(
            margin: Margins.zero,
            padding: HtmlPaddings.zero,
          ),
        },
        // Missing required callbacks for images/links
        onLinkTap: (url, context, attributes, element) {
          // Implementation required
        },
      ),
    );
  }
}

How Senior Engineers Fix It

Migrate away from flutter_html for full-featured email rendering. Senior engineers choose a renderer that matches the requirement:

Use flutter_widget_from_html_core (FWH): A modern, actively maintained package specifically designed to handle web content. It supports Flexbox, reliable images, and better CSS compliance.
Use webview_flutter (Hybrid Approach): Wrap the HTML content in a native WebView. This provides 100% rendering fidelity (what you see in Gmail web) but adds overhead and potential security concerns (XSS).
Implement a Sanitizer: Before passing data to any renderer, use a library like flutter_clean_architecture or custom regex to strip <script> tags and wrap images in a permission-checking proxy.

Refactored Solution (Using flutter_widget_from_html_core):

import 'package:flutter/material.dart';
import 'package:flutter_widget_from_html_core/flutter_widget_from_html_core.dart';

class FixedEmailRenderer extends StatelessWidget {
  final String htmlContent;

  const FixedEmailRenderer({super.key, required this.htmlContent});

  @override
  Widget build(BuildContext context) {
    return HtmlWidget(
      htmlContent,
      // This package handles modern CSS and images much better
      customStylesBuilder: (element) {
        // specific fixes for email quirks can go here
        return null;
      },
      onErrorBuilder: (context, element, error) {
        return Text('Error loading part: $error');
      },
    );
  }
}

Why Juniors Miss It

Package Trust: They assume a package named flutter_html is a “drop-in” solution for rendering HTML without realizing its severe limitations.
Misdiagnosis: They often blame their own HTML sanitization logic (_sanitizeHtml) instead of the rendering engine itself.
Debugging Blindspots: Without inspecting the actual output (e.g., converting the HTML to a WebView to see if it renders there), they cannot distinguish between “invalid HTML” and “unsupported HTML features.”
Ignoring Package Metrics: They don’t check GitHub issue trackers or package maintenance status (e.g., flutter_html has largely been superseded by newer alternatives).