Flagged properties have another issue: they don't solve the problem of proprietary extensions to CSS that become mainstream. If a given vendor implements for its own usage a proprietary feature that is so important to them, internally, they have to "unflag" it, you can be sure some users will start using it if they can. The spread of such a feature remains a problem, because it changes the delicate balance of a World Wide Web that should be readable and usable from anywhere, with any platform, with any browser.
I think the solution is in the hands of browser vendors: they have to consider that experimental features are experimental whetever their spread in the wild. They don't have to care about the web sites they will break if they change, update or even ditch an experimental or proprietary feature. We have heard too many times the message « sorry, can't remove it, it spread too much ». It's a bad signal because it clearly tells CSS Authors experimental features are reliable because they will stay forever as they are. They also have to work faster and avoid letting an experimental feature alive for more than two years.
Emphasis is mine on this last part. Yes it's a very bad signal. And check what was said yesterday.
@AlfonsoML And we will always support them (unlike some vendors that remove things at will). So what is the issue?
This is the issue in terms of Web Compatibility. It's what I was precisely saying that implementers do not understand the impact it has.