Just paste the JS file into your console while having your "exam results" window open and then run the python script on the resulting JSON

average-calculator.py

import json

subjects = json.load(open("znamky.json"))

subjects = [subject for subject in subjects if subject["finished"]]
subjects = [
    {**subject, "marked_attempts": [attempt for attempt in subject["attempts"] if attempt["outcome"].isnumeric()]}
    for subject in subjects
]
subjects = [
    {**subject, "last_attempt": subject["marked_attempts"][-1]}
    for subject in subjects
    if len(subject["marked_attempts"]) > 0
]

weighted_last = sum(int(subject["last_attempt"]["outcome"]) * subject["credits"] for subject in subjects) / sum(subject["credits"] for subject in subjects)
unweighted_all = sum(int(a["outcome"]) for subject in subjects for a in subject["marked_attempts"]) / sum(len(subject["marked_attempts"]) for subject in subjects)
unweighted_last = sum(int(subject["last_attempt"]["outcome"]) for subject in subjects) / len(subjects)
print(f"{weighted_last = }, {unweighted_all = }, {unweighted_last = },")

sis-mark-extractor.js

const data = [...document.querySelectorAll(".row1, .row2")].map(e => {
	const attempts_numbers = e.children[5].innerText.split("\n").map(i => parseInt(i))
	const attempts_outcomes = e.children[6].innerText.split("\n")
	const attempts_dates = e.children[7].innerText.split("\n")
	const attempts_examiners = e.children[8].innerText.split("\n")
	return {
		year: e.parentElement.children[0].innerText,
		semester: e.children[0].children.length > 0 ? "Z" : "L",
		code: e.children[2].innerText,
		name: e.children[3].innerText,
		exam_type: e.children[4].innerText,
		attempts: [...attempts_numbers.keys().map(i => ({
			number: attempts_numbers[i],
			outcome: attempts_outcomes[i],
			date: attempts_dates[i],
			examiner: attempts_examiners[i]
		}))],
		requiredness: e.children[9].innerText,
		credits: parseInt(e.children[10].innerText),
		finished_for_semester: e.children[11].innerText === "Splněno",
		finished: e.children[12].innerText === "Splněno",
	}
})


const hiddenElement = document.createElement('a');
const blob = new Blob([JSON.stringify(data)], {type: "application/json"})
hiddenElement.href = window.URL.createObjectURL(blob)
hiddenElement.download = 'znamky.json';
hiddenElement.click();