An open HTML tag only posed a problem in the section where it was, without having any effect on the others. Today, it can have an effect on a whole page.
Example: https://fr.wikipedia.org/w/index.php?title=Utilisateur:Lofhi/Test&oldid=147832002 with an <u> tag unclosed.
As a result, the history of a page displays these contributor errors without the possibility to ignore them as before.