testbed/Textualize__textual/docs/examples/guide/widgets/hello02.py

from textual.app import App, ComposeResult, RenderResult
from textual.widget import Widget


class Hello(Widget):
    """Display a greeting."""

    def render(self) -> RenderResult:
        return "Hello, [b]World[/b]!"


class CustomApp(App):
    CSS_PATH = "hello02.css"

    def compose(self) -> ComposeResult:
        yield Hello()


if __name__ == "__main__":
    app = CustomApp()
    app.run()