The note should be much bigger. And __proto__ really isn't "supported" anywhere: it's an implementation detail. It's been deprecated in Mozilla, and Opera used not to have it (as recently as 2009, though there is now an __proto__ property on all objects).
> Let’s take a basic figure of an object with which we will operate in the following descriptions. A prototype of an object is referenced by the internal [[Prototype]] property. However, in figures we will use __<internal-property>__ notation, and in particular for the prototype object: __proto__ (that’s moreover, is a real, but non-standard, implementation in some engines, e.g. SpiderMonkey).
It uses __proto__ but makes quite clear that the property is just an implementation detail of the graphs.
I would recommend a note similar to Dmitry Soshnikov's in his Javascript: The Core: http://dmitrysoshnikov.com/ecmascript/javascript-the-core/#a...
> Let’s take a basic figure of an object with which we will operate in the following descriptions. A prototype of an object is referenced by the internal [[Prototype]] property. However, in figures we will use __<internal-property>__ notation, and in particular for the prototype object: __proto__ (that’s moreover, is a real, but non-standard, implementation in some engines, e.g. SpiderMonkey).
It uses __proto__ but makes quite clear that the property is just an implementation detail of the graphs.