Hello world com HTML

from flask import Flask
app = Flask(__name__)


@app.route('/')
def index():
    return '<html><body><h1>Hello World</h1></body></html>'


if __name__ == '__main__':
    app.run(debug=True)

Gerando HTML

from flask import Flask, render_template
app = Flask(__name__)


@app.route('/')
def index():
    return render_template('hello.html')


if __name__ == '__main__':
    app.run(debug=True)