Web devlopment flask and local (url_for)

I would like to develop web applications with flask which are also running on my local system without using flask.

I am searching for something like:

<link href="" rel="stylesheet" type=text/css>

if link above is not working, use: <link href="../static/bootstrap/dist/css/bootstrap.min.css" rel="stylesheet">

I also tried the code below and various other lines. However, it didn't work.

<link href="" onerror="this.onerror=null;this.href='../static/bootstrap/dist/css/bootstrap.min.css';"rel="stylesheet">

I would like to use this for 20 different css and around 30 diffrent js sources so I am searching for a simple solution which is easy to maintain. Do you have an idea? Can you help?

Thanks, Denis