POST requests not properly handled

I have a collection of CSS code to be validated and was sending GET requests with url encoded code as a text query parameter. For a large fraction of them, it was successful, but sometimes validation threw an error since the text was too long to be handled by GET. So, I've tried to send POST requests with the following variables in Python:

import requests

URL = 'http://localhost:8080/css-validator/validator'
HEADERS = {'Content-Type': 'text/css; charset=utf-8'}
PARAMS = {
  'output': 'json',
  'warning': 2,
  'profile': 'css3svg'
}
code = 'STRING_OF_CSS_CODE'

resp = requests.post(URL, headers=HEADERS, params=PARAMS, data=code)
print(resp.content) # resp.json() returns a parsing error

, which returned HTML outputs even though json was given. Parsing HTMLs itself is not a problem, but having an HTML output against the json parameter given seems like an unexpected result for the users. Or, if the way I've tried with is incorrect in sending a POST request and there is some documentation about it, that would be very informational.

Start by reproducing the Python POST request against /css-validator/validator with output=json and compare the response format. Trace how the endpoint handles POST bodies and the output parameter; done means valid CSS submitted by POST returns JSON when requested, or the supported request format is documented clearly.