The problem is that the sites that had that moniker tended to be old, badly written and in ignorance of the actual web standard. Firefox's tracker suggests that

The problem is that the sites that had that moniker tended to be old, badly written and in ignorance of the actual web standard. Firefox's tracker suggests that standardization work on the feature is ongoing. So if you implement it now, you're going to be stuck with a Chrome-only version that may or may not end up being the standard. (Usually it does, because Chrome marketshare beats objections over not well thought out features in it, aka the "IE effect")