You are talking about forward compatibility.

JS is backwards compatible: new engines support code using old features.

JS is not forward compatible: old engines don't support code using new features.

Regarding your iPad woes, the problem is not the engine but websites breaking compat with it.

The distinction matters as it means that once a website is published it will keep working. The only way to break an existing website is to publish a new version usually. The XSLT situation is note-worthy as it's an exception to this rule.