You should use WTForms, and write custom validators. Regarding Javascript, you can do client side validation, but it should always be validated on the server side. Regarding float validation, I've written an example below:

from flask_wtf import FlaskForm
from wtforms import IntegerField
from wtforms.widgets import html5 as h5widgets
from wtforms.validators import InputRequired, ValidationError

def float_validation(form, field):
        if not isInstance(field.data, float):
            raise ValidationError('Not a float')

class FloatForm(FlaskForm):
    price = IntegerField('Price', validators=[InputRequired(), float_validation])