This attribute is from MathML 1 and was probably implemented before we ever had MathML reftests. It has been deprecated from MathML 2 in favor of the display attribute (released 16 years ago) [1] and we have been sending deprecation warnings for it since Mozilla 20 (released 6 years ago) [2]. So I believe it is safe to remove it without introducing a new runtime preference.

[1] https://www.w3.org/TR/MathML2/chapter7.html#interf.toplevel
[2] Bug 553917

What's the policy here, should we wait a bit for feedback on the intent to unship thread before landing this?

It's trivial to backout, so I'd just land it and if someone raises up questions or concerns we can re-evaluate.

Pushed by rmaries@mozilla.com:
https://hg.mozilla.org/integration/autoland/rev/9ea2d91e7e91
Remove <math>'s mode attribute. r=emilio

https://hg.mozilla.org/mozilla-central/rev/9ea2d91e7e91

Regarding the note for developers: The "mode" attribute on the <math> element can be replaced with the "display" attribute, substituing the attribute value "display" with the attribute value "block". See https://www.w3.org/TR/MathML2/chapter7.html#interf.toplevel

For backward compatibility with legacy documents, one can use the following polyfill as a Mozilla extension (for users) or as a page script (for authors): https://github.com/mathml-refresh/mathml/issues/5#issuecomment-521525157

Posted site compatibility note: https://www.fxsitecompat.dev/en-CA/docs/2019/mode-attribute-on-math-has-been-removed/

(In reply to Kohei Yoshino [:kohei] (Bugzilla UX) (FxSiteCompat) from comment #7)

Posted site compatibility note: https://www.fxsitecompat.dev/en-CA/docs/2019/mode-attribute-on-math-has-been-removed/

Thanks! I don't think this attribute has ever been implemented in WebKit/Safari, though.

(In reply to Frédéric Wang (:fredw) from comment #8)

I don't think this attribute has ever been implemented in WebKit/Safari, though.

Thank you! Updated the note along with other changes in Firefox 70: https://www.fxsitecompat.dev/en-CA/docs/2019/various-legacy-mathml-features-have-been-deprecated-or-removed/

Updated: https://wiki.developer.mozilla.org/en-US/docs/Web/MathML/Element/math#Attributes
Added to: https://wiki.developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/70#MathML
Compat data: https://github.com/mdn/browser-compat-data/pull/5047