Adding web support to Flutter involved implementing Flutter’s
core drawing layer on top of standard browser APIs, in addition
to compiling Dart to JavaScript, instead of the ARM machine code that
is used for mobile applications. Using a combination of DOM, Canvas, 
and WebAssembly, Flutter can provide a portable, high-quality,
and performant user experience across modern browsers.
We implemented the core drawing layer completely in Dart
and used Dart’s optimized JavaScript compiler to compile the
Flutter core and framework along with your application
into a single, minified source file that can be deployed to
any web server.

While you can do a lot on the web,
Flutter’s web support is most valuable in the
following scenarios:

Progressive Web Application built with Flutter

Flutter delivers high-quality PWAs that are integrated with a user’s
environment, including installation, offline support, and tailored UX.