What is the ECMAScript version used in WebKit / WebView ?

[Maze-fr]

Hello,

I found, with great difficulties, that JavaFX 21 (because LTS...) uses WebKit 615.1, which is based on GTK WebKit 2.38, which uses JavaScriptCore, I think.
But it is impossible for me to know which version of ECMAScript is implemented.
I know that ES 6 is implemented since JDK 9, even before the OpenJDK, but impossible to find any new information about it.
It is obviously important to know about it when opening an URL in a WebView, because some may fail loading.

So... what is the ECMAScript version supported by JavaFX ?