How to remove the header and footer from website using web view package in flutter

You have to call the Javascript inside the "onPageStarted" for hide header & footer .

Checkout the below code

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


void main() {
  WidgetsFlutterBinding.ensureInitialized();
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: ImprintScreen(),
    );
  }
}

class ImprintScreen extends StatefulWidget {
  const ImprintScreen({Key? key}) : super(key: key);

  @override
  State<ImprintScreen> createState() => _ImprintScreenState();
}

class _ImprintScreenState extends State<ImprintScreen> {
  late WebViewController controller;


  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: SafeArea(
          child: WebView(
            initialUrl: "https://getmobie.de/impressum/",
            javascriptMode: JavascriptMode.unrestricted,
            onWebViewCreated: (webViewController) {
              controller = webViewController;
            },
            onPageStarted: (url) {
                print('Website: $url');
                //Hide header & footer
              if (url.contains('https://getmobie.de/')) {
               //Added delayed future method for wait for the website to load fully before calling javascript
                Future.delayed(Duration(milliseconds: 900), () {
                  controller.runJavascriptReturningResult(
                        "document.getElementsByClassName('elementor elementor-7715 elementor-location-header')[0].style.display='none';"
                        "document.getElementsByClassName('elementor elementor-2727 elementor-location-footer')[0].style.display='none';"
                  );
                });
              }
            },
          ),
        )
    );
  }
}

Screenshot of hidden header

Screenshot of hidden Footer

You can use the flutter_inappwebview plugin (I'm the author) and inject an UserScript at UserScriptInjectionTime.AT_DOCUMENT_START to hide or remove HTML elements when the web page loads (check JavaScript - User Scripts official docs for User Scripts details).

Here is a code example using the current latest version 6 (6.0.0-beta.18) with your URL https://getmobie.de/impressum that removes the header and footer HTML elements:

import 'dart:collection';
import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:flutter_inappwebview/flutter_inappwebview.dart';

Future main() async {
  WidgetsFlutterBinding.ensureInitialized();
  if (!kIsWeb &&
      kDebugMode &&
      defaultTargetPlatform == TargetPlatform.android) {
    await InAppWebViewController.setWebContentsDebuggingEnabled(kDebugMode);
  }
  runApp(const MaterialApp(home: MyApp()));
}

class MyApp extends StatefulWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  final GlobalKey webViewKey = GlobalKey();

  InAppWebViewController? webViewController;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: const Text("InAppWebView test"),
        ),
        body: Column(children: <Widget>[
          Expanded(
            child: InAppWebView(
              key: webViewKey,
              initialUrlRequest:
                  URLRequest(url: WebUri("https://getmobie.de/impressum")),
              initialUserScripts: UnmodifiableListView([
                UserScript(source: """
                window.addEventListener('DOMContentLoaded', function(event) {
                  var header = document.querySelector('.elementor-location-header'); // use here the correct CSS selector for your use case
                  if (header != null) {
                    header.remove(); // remove the HTML element. Instead, to simply hide the HTML element, use header.style.display = 'none';
                  }
                  var footer = document.querySelector('.elementor-location-footer'); // use here the correct CSS selector for your use case
                  if (footer != null) {
                    footer.remove(); // remove the HTML element. Instead, to simply hide the HTML element, use footer.style.display = 'none';
                  }
                });
                """, injectionTime: UserScriptInjectionTime.AT_DOCUMENT_START)
              ]),
              onWebViewCreated: (controller) {
                webViewController = controller;
              },
            ),
          ),
        ]));
  }
}