1388910 - remove support for '-moz-user-modify' CSS property

Status: NEW

(Core :: CSS Parsing and Computation, task, P3)

Description

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

Based on https://lists.w3.org/Archives/Public/www-style/2015Apr/0406.html I think we should consider removing support for the -moz-user-modify CSS property.

The original spec for this was:
https://www.w3.org/TR/2000/WD-css3-userint-20000216#user-modify

It seems like the only thing we do with the property is, under some conditions, change whether the caret is visible:
https://searchfox.org/mozilla-central/rev/c329d562fb6c6218bdb79290faaf015467ef89e2/layout/base/nsCaret.cpp#549

This doesn't particularly seem to match the intent of the property, and makes it seem like we should probably remove the caret-related behavior since it doesn't seem to do anything else.

If we remove it, we might need something else to control the caret behavior, though, since the property does appear to be used to control caret visibility in our UA style sheets.

Alternatively, we could change the property to be accessible only from UA style sheets, and stop exposing it to the Web.