Editor's agree the original language was probably overly strict compare to initial intent of it (as judged by the summary in the next sentence after the modified language). But the restrictive language may still match current implementations. So we should write some tests to confirm whether this is true or not before merging this.

I've written some tentative WPT tests in https://jsfiddle.net/jib1/opsv1ugf/ with console.log statements instead of asserts in places, and the results are in. Chrome, Safari and Edge pretty much yield the same results. Firefox only passes the first (existing) test since we're about to update set/getParameters to spec. In Chrome:

(Note the last 3 PASSes illustrate implemented behaviors, not spec compliance)

FAIL createAnswer() attaches to an existing transceiver with a remote simulcast offer: Expected exactly one transceiver - Expected 1, got 2 failed
PASS SRD(simulcastOffer) creates a simulcast transceiver with rids
FAIL SRD(simulcastOffer) creates a simulcast transceiver with scaleResolutionDownBys: Expected scaleResolutionDownBys - Expected 4,2,1, got ,, failed
PASS SRD(simulcastOffer) creates a simulcast transceiver with actives
foo,bar,baz
foo,bar
foo
foo
foo
PASS Remote reoffer can narrow but not (re-)expand simulcast envelope. Never fails.
0,1,2
0,1
0
0
0
PASS Remote reanswer can narrow but not (re-)expand simulcast envelope. Never fails.
0,1
0
0
PASS Remote reanswer altering rids is treated as removal. Never fails.
5/7
PASS 

To summarize:

1. SRD never fails in response, ignoring changes if needed instead
2. remote offers can narrow the simulcast envelope
3. remote answers can narrow the simulcast envelope
4. neither remote offers nor answers can expand the simulcast envelope, but don't fail
5. Once narrowed, the envelope stays narrowed and cannot be re-expanded
6. Once present, rid ids are never removed from an encoding
7. renaming rids in remote answers is treated as layer removal, but doesn't fail

1, 2, 6 and 7 violate the spec I think. We should perhaps assess how web compatible we think it would be to enforce the spec at this point.

4 also violates the spec since it doesn't fail, I think.

These results seem reasonable, in general. So as a goal, it would seem like a good idea to align the spec to the implementations.

If we agree on this, then I think this means the paragraph this PR is modifying instead goes away entirely, because of 1.

@Orphis does this way forward we suggest make sense to you?

I am worried about 7.

In particular, if the offer is 1,2,3 and the answer is 1,foo,2, exactly what is being removed?

If we want to be lenient, I would treat any answer that has changes as erroneous layers and ignore them. Thus:

1,2,3 + 1,2 -> 1,2 (narrowing allowed)
1,2,3 + 1,3,2 -> 1
1,2,3 + 1,foo,2 -> 1
1,2,3 + foo, 1, 2 -> one layer with no RID

And CreateOffer() after 1,2,3 + 1,2 should offer 1,2 (no widening allowed from either side).

The conversation moved to #2762 (comment) where the current suggestion seems to be to fail over 7.