Version pinning? i.e. you specify in your script the version that you will use. Scripts missing the version pinning information run under backwards compatible mode.
It...sorta worked for HTML? Over the long run, old pages could be phased out and now we have a pretty nice situation with HTML5.
This actually works reasonably well in Perl, where you can, for example, stick "use 5.22.2;" in a script and thus enable all the features of that version (and the versions before it) while keeping the features of newer versions disabled, thus ensuring backward compatibility.
It...sorta worked for HTML? Over the long run, old pages could be phased out and now we have a pretty nice situation with HTML5.