How to display html content through flask messages?

Use the safe filter:

<div class="flashes">
  {% for message in get_flashed_messages()%}
    {{ message|safe }}
  {% endfor %}
</div>

For cases where you might want to control the CSS applied depending on the status of message (Success | Error), the following code might be useful:

{% for category, msg in get_flashed_messages(with_categories=true) %}

    <div class="alert {{ category }} alert-dismissible" role="alert">
                            <button type="button" class="close" data-dismiss="alert" aria-label="Close"><span aria-hidden="true">×</span></button>
                           {{ msg|safe }}
    </div>
{% endfor %}

Another way is to call render_template to the external HTML file and passing that to Markup class.

main/routes.py

from flask import render_template, flash, Markup
from . import blueprint

@blueprint.route("/user")
def user():
    flash(Markup(render_template("templates/user.html", name="John Doe"))
    return render_template("templates/home.html")

templates/user.html

<h1>User: {{ name }}</h1>

Where possible, a secure approach is to wrap your string in a Markup object before passing it to the template:

Python code:

from flask import Markup

message = Markup("<h1>Voila! Platform is ready to used</h1>")
flash(message)
return render_template('output.html')

Jinja2 Template:

<div class="flashes">
  {% for message in get_flashed_messages() %}
    {{ message }}
  {% endfor %}
</div>

Using {{message|safe}} will work, but also opens up the door for an attacker to inject malicious HTML or Javascript into your page, also known an an XSS attack. More info here if you're interested.