webp-container-spec.txt: make reserved 0 values a MUST
these were intended as an extension point, but in this version of the
spec they're not defined. if they ever were used leaving them as SHOULD
could result in a source of incompatibility.
@@ -322,7 +322,7 @@ Extended WebP file header:
Reserved (Rsv): 2 bits
-: SHOULD be `0`.
+: MUST be `0`.
ICC profile (I): 1 bit
@@ -348,11 +348,11 @@ Animation (A): 1 bit
Reserved (R): 1 bit
-: SHOULD be `0`.
+: MUST be `0`.
Reserved: 24 bits
-: SHOULD be `0`.
+: MUST be `0`.
Canvas Width Minus One: 24 bits
@@ -466,7 +466,7 @@ Frame Duration: 24 bits (_uint24_)
Reserved: 6 bits
-: SHOULD be 0.
+: MUST be 0.
Blending method (B): 1 bit
@@ -546,7 +546,7 @@ _padded_ chunks as described by the [RIFF file format](#riff-file-format).
Reserved (Rsv): 2 bits
-: SHOULD be `0`.
+: MUST be `0`.
Pre-processing (P): 2 bits
