Chrome apparently allows passing in an empty string as rootMargin, while Firefox throws this exception:

DOMException: IntersectionObserver constructor: rootMargin must be specified in pixels or percent.

This is breaking at least two shopping site pages completely, as they rely on the same (or very similar) scripts from https://*.b2w.io/catalog-statics/js/:

• https://webcompat.com/issues/87280
• https://webcompat.com/issues/88028

The spec seems clear that with zero tokens we should just fall back to the default: https://w3c.github.io/IntersectionObserver/#parse-a-root-margin

Astounding if there's no test for this.

Backed out changeset f8c0561be4db (Bug 1738791) for causing failures in observer-attributes.html CLOSED TREE

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&group_state=expanded&collapsedPushes=842382&selectedTaskRun=dvY-cdOzRhKCyI9NeTXI_w.0&resultStatus=testfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel&revision=f8c0561be4dbd959951c45f761e4ab825416fa82

Log: https://treeherder.mozilla.org/logviewer?job_id=356705925&repo=autoland&lineNumber=11816

Backout: https://hg.mozilla.org/integration/autoland/rev/cfa8fc5e0f8c1d4c0625feabf897230ec47f0816

https://hg.mozilla.org/mozilla-central/rev/d66bfd00b545

FYI FF96 docs work for this can be tracked in https://github.com/mdn/content/issues/10854#issuecomment-987580613 . This is mostly just a release note and addition of a note to the browser compatibility data.