In 8.3.10.3 "ValidateAndApplyPropertyDescriptor",
steps 2 and 3 are:
2. If current is undefined and extensible is false, then return false.
3. If current is undefined and extensible is true, then
...
If you combined them into:
2. If current is undefined, then
a. If extensible is false, then return false.
b. Else,
...
(or
2. If current is undefined, then
a. If extensible is false, then return false.
b. ...
)
then it would be slightly clearer that this chunk of pseudo-code completely handles the cases where current is undefined.
fixed in rev 13 editor's draft