webp-container-spec.txt: remove 'experimental' markers
unknown chunks as a group are well defined in the text
+ remove the Note about stable features as it matched the list given;
there are no WIP features for the format
@@ -53,10 +53,6 @@ document are to be interpreted as described in [RFC 2119][].
Bit numbering in chunk diagrams starts at `0` for the most significant bit
('MSB 0') as described in [RFC 1166][].
-**Note:** Out of the features mentioned above, lossy compression, lossless
-compression, transparency, metadata, color profile and animation are finalized
-and are to be considered stable.
-
Terminology &amp; Basics
------------------------
@@ -269,7 +265,7 @@ An extended format file consists of:
* An optional 'XMP ' chunk with XMP metadata.
- * An optional list of [unknown chunks](#unknown-chunks). _\[status: experimental\]_
+ * An optional list of [unknown chunks](#unknown-chunks).
For a _still image_, the _image data_ consists of a single frame, which is made
up of:
@@ -705,7 +701,7 @@ XMP Metadata: _Chunk Size_ bytes
Additional guidance about handling metadata can be found in the
Metadata Working Group's [Guidelines for Handling Metadata][metadata].
-#### Unknown Chunks _\[status: experimental\]_
+#### Unknown Chunks
A RIFF chunk (described in [this](#terminology-amp-basics) section) whose _chunk
tag_ is different from any of the chunks described in this document, is
