Add spec convention about order of operations

Triggered by the discussion in tc39/proposal-temporal#2377 (comment)
Arguments should consistently be processed in order.

spec-conventions.md

@@ -15,4 +15,32 @@
 
 ## Implementations of functions and methods
 
+### Order of observable operations
+
+When writing the algorithm steps for the implementation of a JS built-in
+function or method, this order should be followed:
+
+1. If applicable, process and validate the receiver.
+1. Process and validate each argument, in order.
+1. Perform validation of any other preconditions, if any.
+1. Perform the actual work of the function.
+
+Validating the arguments includes type checking, but also unobservable things
+like supplying default arguments.
+So, for a fictitious `addTwoNumbersThatArentTooFarApart(a, b = 42)` function,
+the algorithm steps could look like this:
+
+> 1. Let _firstOperand_ be ℝ(? ToNumber(_a_)).
+> 1. If _b_ is **undefined**, then
+>     1. Let _secondOperand_ be 42.
+> 1. Else,
+>     1. Let _secondOperand_ be ℝ(? ToNumber(_b_)).
+> 1. If _firstOperand_ - _secondOperand_ > 10, then
+>     1. Throw a **RangeError** exception.
+> 1. Return 𝔽(_firstOperand_ + _secondOperand_).
+
+The first three steps process and validate each argument in order; Step 4
+validates the other precondition that the numbers aren't too far apart; and
+Step 5 does the actual work.
+
 ## Normative or editorial?