What to use as a front end for a python back end?

we are using a Python(flask) in the backend and making a reservation website.

I am wondering what will be most suitable for the front end?

Because i need to be able to pass variables to python and the database( such as entries, time selections and all that) so i am wondering if HTML/CSS/JS will work or I need to go with PHP.

If anyone can suggest any ideas i appreciate it.

Thanks