Summary: Motion is a key component of modern web design, and <marquee> is the 
premier... just kidding. Gecko currently ships <marquee> as an HTMLDivElement 
with the web-exposed properties attached via in-content XBL [0][1]. As part of 
the process of removing in-content XBL, I intend to implement and ship 

This will allow us to expose properties on the element through WebIDL instead 
of through `<xbl:property exposeToUntrustedContent="true">`. This will lead to 
behavior changes with the current implementation 
(, although those 
changes will generally bring Firefox into compliance with the spec and with 
other implementations - many currently failing web platform tests should start 
passing [2][3].

The event handlers (onbounce, onfinish, onstart) will still be implemented in 
XBL for the time being - the actual XBL removal will happen in subsequent bugs.

There's an open question about how to treat the `loop` property. As outlined at, the spec defines a 
no-op when setting `loop` to a number that is <= 0 and not -1, which is similar 
to what our current implementation does. In this case Chrome, Safari, and Edge 
throw, so we may want to adapt this behavior.


Link to standard:

Platform coverage: All

Target Release: 65

Preference behind which this will be implemented: None

Is this feature enabled by default in sandboxed iframes? Yes

DevTools bug: None

Do other browser engines implement this?
- Chrome: since version 1
- Edge: yes
- IE: since version 2
- Safari: since version 1.2


Is this feature restricted to secure contexts? No



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