[CSS] Implement margin at-rules

[dipikabh]

Acceptance criteria

• [ ] The listed features are documented sufficiently on MDN
• [ ] BCD is updated
• [ ] Interactive example and data repos are updated if appropriate
• [ ] The content has been reviewed as needed

For folks helping with Firefox related documentation

• [ ] Set bugs to dev-doc-complete
• [ ] Add entry to Firefox release notes for enabled/preview features
• [ ] Add/remove entry to Firefox experimental features page for preference/released features

Related Gecko bugs

• Bug 1833466: Implement CSSMarginRule
• Related - Bug 856371: [css3-page] implement @page margin boxes
• Preference: layout.css.margin-rules.enabled

Spec:

• https://drafts.csswg.org/css-page-3/#margin-at-rules
• https://drafts.csswg.org/cssom/#the-cssmarginrule-interface

Related pages on MDN

• https://developer.mozilla.org/en-US/docs/Web/CSS/@page#margin_at-rules
• https://developer.mozilla.org/en-US/docs/Web/API/CSS_Object_Model

Other

• [ ] Check content open issues to see if any pertain to the subject matter. If there are any that can be closed because of the work, do so. If there are any that can be fixed relatively quickly because of the knowledge from completing this issue and you have time, feel free to go ahead and fix them.

[pepelsbey]

• BCD: https://github.com/mdn/browser-compat-data/pull/23167
  • It seems like the style property is not supported. Trying to get a response from the implementor
• Content: WIP

[pepelsbey]

The feature is only partially implemented behind the flag. Moved the task to the backlog.