MAL has added a new "Notes" feature on the list. It's similar to tags but a bit different. Basically they are notes under your titles which you can easily edit by simply clicking on them. But you can also use BBcode in these notes, so that means you can add custom text, images, and even Youtube videos to your list easily under every title!
You can turn Notes on through this page
https://myanimelist.net/editprofile.php?go=listpreferences
At the right of Show Columns under Common.
From there, you can edit your anime notes by simply clicking on them under each title, or clicking on the Edit link and editing them there. It would make sense for many users to move their Tags to Notes since Tags were used as notes in the past and for mini-reviews; maybe MAL will make it easy to do so in the future since doing so would be time-consuming.
btw, the Notes feature is actually an update of the Comments feature. The comments are normally hidden and found by clicking the More button next to your titles. Turning on Notes unhides all comments and moves all them under their titles! Many users probably do not know the Comments feature even existed all this time, but it's how we've been adding "Music Players" (simply a Youtube music video pinned to your list haha) and some special tricks to list designs for years, although it was very complicated and annoying. Now these tricks should be much easier! :D
I'm using notes currently on my list (September 8th).
https://myanimelist.net/animelist/Shishio-kun?status=2
Notes Examples including with BBcode
Fixed Music Player
You can tell from the cell phone the anime is 2000s era!
To add a Youtube video to an anime or fix a music player to your page, you need to:
1) Turn notes on and save:
https://myanimelist.net/editprofile.php?go=listpreferences
2) Add a Youtube player to one of your notes on your list, simply click the notes and add BBcode for it

You can learn how to use BBcode here
https://myanimelist.net/forum/?topicid=496203
3) The video should be visible on your list. Now you can add the CSS to fix the player if you want:
div.notes-5525{
position: fixed;
top: 0;
left: 0;
}
Important:
You must replace 5525 with the number of the anime you added the Youtube player to. The anime's # can be found by visiting the page and checking the URL, the number will be in slashes after "anime" or "manga":
https://imgur.com/g0h5fLr
The code above is for 07 Ghost on my completed page.
https://myanimelist.net/anime/5525/07-Ghost