El.PropSourceElement properties.
The type for element properties of type 'a.
bool n is a property named n accessed as a bool and returning false on undefined.
int n is a property named n accessed as an int and returning 0 on undefined.
float n is a property named n accessed as a float and returning 0. on undefined.
jstr n is the property named n accessed as a Jstr.t and returning Jstr.empty on undefined.