One of the major selling points of Flutter is that it's built with Dart.
That means it can be compiled to any target platform (including the web!)
Starting earlier in 2019 there has been ("beta") support for Web in Flutter: https://flutter.dev/web

Official getting started guide: https://flutter.dev/docs/get-started/web

@Nash0x7E2 wrote this overview of "Ins and Outs of Flutter Web":
https://medium.com/flutter-community/ins-and-outs-of-flutter-web-7a82721dc19a
@zubairehman has written this great post on getting started building a Web App with Flutter:
https://itnext.io/flutter-for-web-c75011a41956
@mikegazzaruso wrote Flutter For Web: a concrete example on flutter-community:
https://medium.com/flutter-community/flutter-for-web-a-concrete-example-4ed293545008
Sample web app: https://github.com/kenreilly/flutter-web-example

Todo

• install Flutter on localhost
• Follow the official instructions to build a "hello world" web application
• Deploy it to a conventional web hosting service e.g. Fly.io to test the process.

https://youtu.be/tot-R_q5-0o?t=6