Original text:
The Frame Type field uses a variable length integer encoding (see integer-encoding) with one exception.  To ensure simple and efficient implementations of frame parsing, a frame type MUST use the shortest <del> possible encoding.  Though a two-, four- or eight-byte encoding of the frame types defined in this document is possible, the Frame Type field for these frames is encoded on a single byte.  For instance, though 0x4001 is a legitimate two-byte encoding for a variable-length integer with a value of 1, PING frames are always encoded as a single byte with the value 0x01.  An endpoint MAY treat the receipt of a frame type that uses a longer encoding than necessary as a connection error of type PROTOCOL_VIOLATION. </del>  13.

Reviewer comments:
Fixes issue
Thanks for this. It's a lot clearer this way.
restricts frame type varint encoding: The relevant sentence is: This means that the restriction may not apply to frames defined by QUIC extensions (such as the frame in the Delayed Acks_ extension). Must each extension spell out its own frame type encoding rules? It would be easier to change the transport draft to extend the minimal encoding restriction to not-as-yet-specified frames as well.
I read the first sentence as applying to all QUIC frame types, core or extensions. The examples confuse the matter by explicitly mentioning the core frames. Section 19.21 contains some guidance for extension frames, adding a pointer back to the encoding rules from would be a good editorial change.
The sentence I highlighted talks about frames in this document and then these frames, effectively overriding what the first sentence may imply.
yes I think we agree :)
I think we all agree that it can be made more explicit. The first sentence says that the rule for all frame types is that they be minimally encoded. Applying that principle to all frame types defined in this document means that they get one-byte encodings. We should clarify that it's an application of the rule, not a limitation of it.
Why did we have this rule in the first place? It just seems to complicate things in unexpected ways.
NAME See . IIRC the rationale for using minimal varint encoding is that stacks that support only the standardized frames can live with a simpler dispatch scheme that takes one byte as an input (regardless of the dispatch method being a table or a switch clause or whatever). While I do not have a strong opinion regarding that requirement, I'd prefer not revisiting the discussion.
PR has been merged -- closing bug.

Revised text:
The Frame Type field uses a variable length integer encoding (see integer-encoding) with one exception.  To ensure simple and efficient implementations of frame parsing, a frame type MUST use the shortest  <ins> possible encoding.  For frame types defined in this document, this means a single-byte encoding, even though it is possible to encode these values as a two-, four- or eight-byte variable length integer. For instance, though 0x4001 is a legitimate two-byte encoding for a variable-length integer with a value of 1, PING frames are always encoded as a single byte with the value 0x01.  This rule applies to all current and future QUIC frame types.  An endpoint MAY treat the receipt of a frame type that uses a longer encoding than necessary as a connection error of type PROTOCOL_VIOLATION. </ins> 13.