[heycam/webidl] Proposed solution to make grammar changes less disruptive (#477)

### Goal ###
Allow faster and easier changes to the grammar while being less disruptive to specs and tools relying on WebIDL.

### Issue ###
Grammar changes are particularly problematic as removed syntax causes parsers to fail. This has the following consequence:

- Previously passing idlharness tests start to fail or error.
- ReSpec specs rendered in the client suddenly start completely failing.
- New builds with Bikeshed require updating the WebIDL.

Working around those issues creates yet another level of problems:

- Avoiding to update the idlharness parser to avoid breaking previous tests incentives editors to keep to the old syntax.
- Parsers may decide to keep supporting older syntax, thus creating situations where certain syntax is supported by Bikeshed, but not by ReSpec, or by ReSpec but not by idlharness, which is really confusing for editors.

### Proposed solution ###

Don't remove grammar right away. Instead, mark grammar as deprecated for a given period of time (6 months?). Have all the tools (parsers, idlharness, ReSpec, Bikeshed) implement the same version of the grammar (with the same deprecation warnings) and sync dropping off support.

Add information in the spec as to when and what tools may/should emit as warning for deprecated syntax.

### Immediate targets ###

- I think `serializer` has been mostly fixed, so it's probably not worth adding it back to the spec and marking it as deprecated (also because it seems like a lot of work). @gsnedder, @foolip, can you confirm?
- `implements` is a great candidate, however.

Would love to hear @plinss's, @marcoscaceres's, @tabatkins's, @bzbarsky's, @annevk's and  @domenic's thoughts on the topic.


-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/heycam/webidl/issues/477