Yes you can do that. Here are the steps:
See the example here, see how it uses HTML strings such as kNavigationExamplePage:
https://pub.dev/packages/webview_flutter/example
const String kNavigationExamplePage = '''
<!DOCTYPE html><html>
<head><title>Navigation Delegate Example</title></head>
<body>
<p>
The navigation delegate is set to block navigation to the youtube website.
</p>
<ul>
<ul><a href="https://www.youtube.com/">https://www.youtube.com/</a></ul>
<ul><a href="https://www.google.com/">https://www.google.com/</a></ul>
</ul>
</body>
</html>
''';
Future<void> _onNavigationDelegateExample(
WebViewController controller, BuildContext context) async {
final String contentBase64 =
base64Encode(const Utf8Encoder().convert(kNavigationExamplePage));
await controller.loadUrl('data:text/html;base64,$contentBase64');
}
What you need to do is to read this string from a File instead. This answer provides detailed steps on how to do that. Instead of a text file, you will read an HTML file. Later you will use it instead of the kNavigationExamplePage string.
Flutter - Read text file from assets
Edit: If you are using flutter_inappwebview, it seems it even has a function that uses your asset files directly: https://pub.dev/documentation/flutter_inappwebview/latest/flutter_inappwebview/InAppWebViewController/loadFile.html