Why isn't JavaScript versioned?

Everyone knows JavaScript has some -- or maybe even lots? -- of shortcomings.

Probably the most (in)famous one, might be the triple equals. While the double equals with its type conversion might be useful, its behavior is not obvious up front, so nearly all equality checking uses the triple equals.

In addition, there are also lots of inconsistencies in the language, for example the existence of window.parseInt and Number.parseInt.

The reason these issues exist, is to be as backwards compatible, so that older websites do not break. It seems to me that a much better solution is available.

Why not add, say a “version” attribute to script tags? This way, modern websites which use a modern JavaScript spec, would be able to use a better version of the language without these issues.