Как показать html-строку во флаттере текстового виджета

У этого плагина нет никаких проблем, я просто создал образец с вашим HTML, и он отлично работает. Попробуйте заменить на приведенный ниже фрагмент и посмотрите, работает ли это.

dependencies:
  flutter_html: ^0.8.2
        

и импорт и код для рендеринга html

import 'package:flutter_html/flutter_html.dart';
import 'package:html/dom.dart' as dom;


 body: new Center(
            child: SingleChildScrollView(
              child: Html(
                data: """
                <div>Follow<a class='sup'><sup>pl</sup></a> 
                  Below hr
                    <b>Bold</b>
                <h1>what was sent down to you from your Lord</h1>, 
                and do not follow other guardians apart from Him. Little do 
                <span class='h'>you remind yourselves</span><a class='f'><sup f=2437>1</sup></a></div>
                """,
                padding: EdgeInsets.all(8.0),
                onLinkTap: (url) {
                  print("Opening $url...");
                },
                customRender: (node, children) {
                  if (node is dom.Element) {
                    switch (node.localName) {
                      case "custom_tag": // using this, you can handle custom tags in your HTML 
                        return Column(children: children);
                    }
                  }
                },
              ),
            ),
          )

Не используйте flutter_html_view, чтение в документация:

Supported Tags

• p
• em
• b
• img
• video
• h1,
• h2,
• h3,
• h4,
• h5,
• h6
• Note

This plugin converts some of the html tags to flutter widgets This plugin does't support rendering full html code (there is no built in support for web rendering in flutter)

Так что он просто плохо отображает ваш html, потому что не может.

Но вы можете найти и другие пулугины, например flutter_html.

https://github.com/Sub6Resources/flutter_html

и дайте им попробовать, чтобы увидеть, лучше ли они работают.

ОБНОВИТЬ

В pubspec.yaml я добавил

dependencies:
  flutter_html: ^0.8.2

а мой main.dart

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


void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;
  var html = "Follow<a class='sup'>pl</a> what was sent down to you from your Lord, and do not follow other guardians apart from Him. Little do <p class='h'>you remind yourselves</p><a class='f'><sup f=2437>1</a>.";


  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Html(
              data: html,
              ),
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

Используйте плагин flutter_webview_plugin

пример:

new WebviewScaffold(
              url: new Uri.dataFromString('<html><body>hello world</body></html>', mimeType: 'text/html').toString()

Веб-просмотр внутри настраиваемого прямоугольника

final flutterWebviewPlugin = new FlutterWebviewPlugin();

flutterWebviewPlugin.launch(url,
  fullScreen: false,
  rect: new Rect.fromLTWH(
    0.0,
    0.0,
    MediaQuery.of(context).size.width,
    300.0,
  ),
);

Я только что сделал следующее, и он отлично работает.

1. Добавьте flutter_html в свой файл pubspec.yaml.

dependencies:
  flutter:
    sdk: flutter
  flutter_html: ^0.8.2 

2. Выполните следующую команду, чтобы обновить пакеты.

flutter pub get

3. Импортировать flutter_html

import 'package:flutter_html/flutter_html.dart';

4. Замените текстовый виджет на виджет HTML.

   child: 
   // Text(
   //   "Hello Programmer",
   //   style: TextStyle(fontSize: 18),
   // ),
   Html(data:"<p>Hello <b>Flutter</b><p>"),

Добавьте в файл pubspec.yaml следующее:

dependencies:
  flutter_html:

Поддерживаемые в настоящее время HTML-теги:

a, abbr, аббревиатура, адрес, статья, в сторону, b, bdi, bdo, big, цитата, тело, br, подпись, цитировать, код, данные, dd, del, dfn, div, dl, dt, em, figcaption, рисунок, нижний колонтитул, h1, h2, h3, h4, h5, h6, header, hr, i, img, ins, kbd, li, main, mark, nav, noscript, ol, p, pre, q, rp, rt, ruby, s, samp, section, small, span, strike, strong, sub, sup, table, tbody, td, template, tfoot, th, thead, time, tr, tt, u, ul, var

Пример использования:

Column(
   mainAxisAlignment: MainAxisAlignment.start,
   crossAxisAlignment: CrossAxisAlignment.start,
   children: [
      new Html(
         data: "<b>Welcome</b>,
          defaultTextStyle: TextStyle(fontSize: 15),
       ),
     ],
)

В pubspec.yml добавьте следующее:

dependencies:
  flutter_html:

Затем запустите:

flutter clean && flutter pub get

Наконец, добавьте такой код:

import 'package:flutter_html/flutter_html.dart';
.
.
.
body: Center( Html(
      data: """
            <div>This is the start of a div
              <a class='sup'><sup>a sup</sup></a> 
              and after sub
              <b>Bold</b>
              <h1>This is a header with number 1557</h1> 
              A text and 
            <span class='h'>some stuff</div>
            """,
    ),)

До сих пор он отлично работал как на мобильных устройствах, так и в Интернете.