update SSZ static doc to reflect options in test conditions

specs/test_formats/ssz_static/core.md

@@ -40,10 +40,14 @@ The same value as `serialized.ssz`, represented as YAML.
 ## Condition
 
 A test-runner can implement the following assertions:
-- Serialization: After parsing the `value`, SSZ-serialize it: the output should match `serialized`
+- If YAML decoding of SSZ objects is supported by the implementation:
+    - Serialization: After parsing the `value`, SSZ-serialize it: the output should match `serialized`
+    - Deserialization: SSZ-deserialize the `serialized` value, and see if it matches the parsed `value`
+- If YAML decoding of SSZ objects is not supported by the implementation:
+    - Serialization in 2 steps: deserialize `serialized`, then serialize the result, 
+       and verify if the bytes match the original `serialized`.
 - Hash-tree-root: After parsing the `value` (or deserializing `serialized`), Hash-tree-root it: the output should match `root`
     - Optionally also check `signing_root`, if present.
-- Deserialization: SSZ-deserialize the `serialized` value, and see if it matches the parsed `value`
 
 
 ## References