I had to look up what it's about:
The <marquee> HTML element is used to insert a scrolling area of text.
if it was enabled, I am sure campaign managers would use this tag for their signature campaigns.
Let's not do that! Theymos disabled animated avatars for exactly the same reason: blinking things drawing your attention is very annoying. Every time I use a new browser without adblocker, I'm reminded of how terrible websites are with everything blinking and moving. I don't want that in a book, and I don't want it on my screen.