You could also modify Opera's Accept header that it sends with https://chrome.google.com/webstore/detail/modify-header-value-http/cbdibdfhahmknbkkojljfncpnhmacdek.
You'd add a new rule in options:
Set the URL field value to * to cover all sites (set it to the site you want).
Check the "Modify" box.
In the Header Name field, enter Accept
In the Header Value field, put text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Click the + to add the rule.
What that does is remove image/webp from Opera's original of:
so that the site doesn't think Opera supports webp. Sites can choose to ignore it, but this method has worked for some problem sites in the past.
Re: Not accepting .webm or .mp4 for custom animated wallpaper anymore?
Here is a workaround for making your own animated wallpaper.
Go to https://addons.opera.com/da/wallpapers/details/rain-in-gta-v/. right-click on "add to Opera" and chose "save linked content as".
Convert the background folder your've downloadet from .avzip to .zip
Delete video.webm inside the zip folder
Rename your own .webm video to video.webm and put it in the background zip folder
Go to your /Opera folder and find a folder named 65.0.3467.78 or something like it (If you have more than one then take the newest).
Go to inside the 65.0.3467.78 folder then go to resources\standard_themes
Put your backgroud zip folder in this folder
If you have opera open then close it and reopen it. The background should then be visable under all backgrounds.