This page contains a list of bugfixes, features, or APIs we can't ship (or remove) in Firefox because doing so would break the web for our users.
|-webkit-min-device-pixel-ratio"/"-webkit-max-device-pixel-ratio media query||1237720||Enabling this breaks Google docs icons. Last attempt / backout was March 14, 2018 (1444139)|
|TC39 global proposal||1317422||Broke deezer, flickr, moment.js, Jira|
|Stop dispatching keypress events for non-printable keys||968056||Blocked by bug of Closure framework|
|Start to dispatch keydown and keyup events even during composition||354358||Should be fixed at same time with bug 968056 for saving web app developers' cost|
|Hiding all Gecko specific editing UI||1449564||Collect actual usage first (1452538)|
|Removing window.controllers||1010577||Sites use as a proxy for UA sniffing (possible removal in 1448045)|
|Offline Application Cache||1237782||Big players still use this for various high profile sites (e.g. Microsoft), alternative: Service Workers|
|Clear window.name on cross-origin nav||Not certain this is unshippable - safari shipped it years ago, and Chrome is trying|
|add me||add me||add me|
See also https://html.spec.whatwg.org/#concept-navigator-compatibility-mode and in particular navigator.taintEnabled() and navigator.oscpu.