Intent to prototype: CSS Margin rules

[Emily McDonough]

Summary: CSS margin rules provide a way to style and set the content of page margins, including headers and footers, for paginated layout.
Bug: https://bugzilla.mozilla.org/856371
Specification: https://drafts.csswg.org/css-page-3/#margin-at-rules
Platform coverage: All
Preference: Developed behind layout.css.margin-rules.enabled, defaults to false.
Devtools bug: None currently. We don't currently have a devtools system for managing/inspecting/editing printing-only layout features, such as those in page-rules or margin-rules.
Other Browsers: Not implemented in WebKit or Blink. Fully implemented by PrinceXML.
web-platform-tests: https://wpt.fyi/results/css/cssom/idlharness.html?label=experimental&label=master&aligned