I very much treat this site as a testing ground for my learning and therefore generally start using new features as soon as they are newly available in Baseline. Hence, your browser, which hasn't been updated in over a year, is having issues rendering all the modern features. You might want to consider updating your browser, as it has some known security vulnerabilities.
You are right, it is a bit dated. The website works with Firefox. I will update Vivaldi later and see if it works. I will get back to you if it does not work with the latest version of Vivaldi, which is Chromium-based.
