[CSS] Motion Path docs tasks, Fx 70

[chrisdavidmills]

Trello: https://trello.com/c/Ef3V38Yh/212-css-motion-path-fx-70

Acceptance criteria

1. • [motion-1] Support <offset-anchor>

[chrisdavidmills]

offset-anchor page created: https://developer.mozilla.org/en-US/docs/Web/CSS/offset-anchor

[chrisdavidmills]

Note added to Fx 70 rel notes: https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/70#CSS

[chrisdavidmills]

Other updates:

• compat data added: adding the css offset-anchor property browser-compat-data#4837
• interactive example added: adding offset anchor example interactive-examples#1442
• group name fixed, to stop some links breaking: changing CSS Motion group to CSS Motion Path data#388
• aforementioned release note removed again because I then realised it is behind a pref!

[BorisChiou]

One thing to note: for <position> value, we have retired the 3-value syntax already (except for <background-position>), in Bug 1559276.

The resolved spec issue is w3c/csswg-drafts#2140.

[BorisChiou]

This means these examples:

offset-anchor: bottom 10px right;
offset-anchor: top right 10px;

may need to be updated.

For other parts, looks good to me. Thanks for the documentation.

[chrisdavidmills]

Cool, thanks for the review @BorisChiou !

I've removed the 3-value examples, and added a note to https://developer.mozilla.org/en-US/docs/Web/CSS/offset-anchor#Values.

We are actually documenting that change separately, as part of #2084

[BorisChiou]

Cool! Thanks!

[jmswisher]

Should have been closed a while ago