Localhost
A lot of apps are built using hybrid app development frameworks that use one big WebView for providing app developers a native wrapper and some plugins for their Web app. The web content is often bundled and served within the native app. For a couple of reasons using the file: protocol to access web content is no longer an option and WebViews provide APIs to host content.
See more in usage and challenges.
WebView2
Windows
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WebView2 has a variety of ways to work with local content - intercepting web resources as they’re requested, mapping a hostname to a folder on the user’s filesystem, or registering a custom URL scheme. You can find details on all of these in WebView2’s documentation for working with local content in WebView2 apps.