First functional version

main.js

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+
+
+// Returns the ISO week of the date.
+Date.prototype.getWeek = function () {
+    var date = new Date(this.getTime());
+    date.setHours(0, 0, 0, 0);
+    // Thursday in current week decides the year.
+    date.setDate(date.getDate() + 3 - (date.getDay() + 6) % 7);
+    // January 4 is always in week 1.
+    var week1 = new Date(date.getFullYear(), 0, 4);
+    // Adjust to Thursday in week 1 and count number of weeks from date to week1.
+    return 1 + Math.round(((date.getTime() - week1.getTime()) / 86400000
+        - 3 + (week1.getDay() + 6) % 7) / 7);
+}
+
+const getCoursePage = (course) => {
+    return fetch(`https://www.fit.vut.cz/study/course/${course}/.cs`)
+        .then(response => response.text())
+        .then(html => {
+            const parser = new DOMParser();
+            const doc = parser.parseFromString(html, 'text/html');
+            return doc;
+        })
+}
+
+const getLectureTitles = (course) => {
+    return getCoursePage(course)
+        .then(page => {
+            const planList = page.querySelectorAll('.b-detail__content ol li')
+            let lectureTitles = []
+            planList.forEach(planElement => {
+                lectureTitles.push(planElement.innerText);
+            })
+            return lectureTitles;
+        })
+}
+
+const getDateFromLectureString = (str) => {
+    const regex = /,\s([0-9]+).\s([0-9]+).\s([0-9]+)/mg;
+    let m;
+    let result = [];
+
+    while ((m = regex.exec(str)) !== null) {
+        // This is necessary to avoid infinite loops with zero-width matches
+        if (m.index === regex.lastIndex) {
+            regex.lastIndex++;
+        }
+
+        // The result can be accessed through the `m`-variable.
+        m.forEach((match, groupIndex) => {
+            result.push(match);
+        });
+    }
+
+    return result.slice(1, 4);
+}
+
+const insertLectureNumbering = async (course) => {
+
+    const lectureTitles = await getLectureTitles(course);
+
+    const lectureList = document.querySelector("ul");
+    const lectures = lectureList.children;
+
+    let prevWeek = -1;
+    let firstSchoolWeek = -1;
+
+    Array.from(lectures).forEach(lecture => {
+        const lectureText = lecture.innerText;
+        const splitDate = getDateFromLectureString(lectureText);
+
+        // (Y, M - 1, D) - JS counts months from 0
+        const date = new Date(splitDate[2], splitDate[1] - 1, splitDate[0])
+        const week = date.getWeek();
+
+        if (firstSchoolWeek === -1) {
+            firstSchoolWeek = week
+        };
+
+        const schoolWeek = week - firstSchoolWeek + 1;
+
+        if (prevWeek !== week) {
+            let titleElement = document.createElement("h2")
+
+            titleElement.innerHTML = "Week " + schoolWeek + ((lectureTitles.length) ? " - " + lectureTitles[schoolWeek - 1] : "");
+
+            lecture.parentElement.insertBefore(titleElement, lecture);
+            prevWeek = week;
+        }
+    });
+}
+
+
+
+const pageNavigation = document.querySelectorAll('tbody tr:nth-child(3) td:nth-child(2) > a')
+const pageNavigationLevel = pageNavigation.length
+
+
+if (pageNavigationLevel == 3) {
+    const courseName = document.querySelector('tbody tr:nth-child(3) td:nth-child(2) > b').innerText
+    const courseID = courseName.substring(0, courseName.indexOf(' '));
+
+    console.log(courseID);
+    insertLectureNumbering(courseID);
+}
+

manifest.json

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+{
+  "manifest_version": 2,
+  "version": "1.0",
+  "name": "VUT FIT video server addon",
+  "description": "VUT FIT video server addon for showing lecture titles and week numbering",
+  "content_scripts": [
+    {
+      "matches": ["*://*.video1.fit.vutbr.cz/*"],
+      "js": ["main.js"]
+    }
+  ],
+  "permissions": ["https://www.fit.vut.cz/*"]
+}