That first feature is known as a " let expression" and it is nonstandard; it was dropped from Firefox 41, and the similarly nonstandard " let block" was dropped from Firefox 44: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let#Non-standard_let_extensions

I was surprised to find that this particular non-standard JS was not mentioned in Kangax's table, but I guess he had to restrict this list to non-standard JS extensions that are supported by multiple engines: https://kangax.github.io/compat-table/non-standard/

If you want to go deeper down the rabbit-hole, and Kangax and MDN haven't satisfied your curiosity, this old reference may tell you about curiosities in older browsers: help.dottoro.com/ljsdaoxj.php

Beyond that, the browser-makers usually document the quirks of their own browsers (MDN is also good about documenting non-Mozilla quirks, but it's not perfect); speaking of quirks, Peter-Paul Koch documents both standard and non-standard DOM methods here: quirksmode.org/dom/

Anyway, these aren't just "not yet" in the standards, but likely "not ever" and you shouldn't use them in your own code.