Though there is the rare case when a browser had supported a feature for a while, and a nasty bug makes its way into public release where you need to hotfix for

Though there is the rare case when a browser had supported a feature for a while, and a nasty bug makes its way into public release where you need to hotfix for a specific version (3D-transforms refusing to work in Firefox 18.0, yet being reported as supported, as an example).