FF101 CSS scroll() function on animation_timeline

[hamishwillee]

FF101 added support for the scroll() CSS functional notation in https://bugzilla.mozilla.org/show_bug.cgi?id=1737918 behind a preference.

I haven't added data for one of these before, but the scroll() appears to only be applicable on an animation-timeline as an option, so I have added it there. I would have made the "key" into scroll() but the linter wouldn't let me, so I followed the pattern of font-variant-alternates and used the description to clarify it as a function - that OK?

Other docs work for this can be tracked in mdn/content#15467

FYI @queengooborg

[hamishwillee]

@queengooborg Can we please merge this?

[hamishwillee]

Note, Chrome also implements this behind a preference, but I don't know how to work out what that preference IS - the associated bug is https://bugs.chromium.org/p/chromium/issues/detail?id=1317765

[queengooborg]

Note, Chrome also implements this behind a preference, but I don't know how to work out what that preference IS - the associated bug is https://bugs.chromium.org/p/chromium/issues/detail?id=1317765

It might actually not -- https://bugs.chromium.org/p/chromium/issues/detail?id=1317765#c53 says that "Making animation-delay a shorthand based on a runtime flag is a bit complicated since our code generation does not support it. That work is therefore deferred to a later CL." But it's not a big concern to me to research, if someone else wants to dig deeper and make a PR, I won't say no! ;P