Aside from the built-in schemes, such as `http` and `qrc`, Qt WebEngine may be extended with custom schemes by creating custom scheme handlers.

This is a simple HTML page loaded from a custom scheme and displayed by a `QWebEngineView`. Even the Quit button below is a standard HTML `<button>` element.

Read the documentation to find out

- How to create a custom scheme handler which serves HTML and handles HTML form submissions.

- How to prevent ordinary web content from accessing the custom scheme.

- How to prevent any other scheme from submitting HTML form data.