First, even if they are not in the source, nothing prevents a script from adding them.
Second, do you experience this in all of the web pages and all of the websites, or only on that W3Schools page?
Third, if you go incognito (Control + Shift + N, on Windows at least) and browse to the same place, do you still experience this?
Fourth (kind of related to the third point, but, still), did you try disabling any extension you installed? When you something strange, it is usually an intended or unintended deed of an extension.