Not just an interpreter – unless there's a standardized subset that one can look for as a web developer then one can't really adopt to it (through either graceful degradation or progressive enhancement). Same reason why eg. Noscript addons creates havoc at times. Web sites expects a certain baseline and when it's not there, they expect that javascript will be disabled altogether and <noscript>-tags to get executed, but just disabling some code prohibits that fallback, making it less usable than fully disabling js.