diff --git a/grammar.w3c-ebnf b/grammar.w3c-ebnf
index 9c9497e4..e7147711 100644
--- a/grammar.w3c-ebnf
+++ b/grammar.w3c-ebnf
@@ -64,7 +64,7 @@ Percentage ::= CompareOperator Space* Numeric '%' (Space+ 'in' Space+ (MyStats |
 
 Cover ::= 'cover' Space+ Numeric '%' (Space+ 'in' Space+ (MyStats | Region))?
 
-FeatureSupport ::= 'supports' Space+ ([a-z0-9] | '-')+
+FeatureSupport ::= (('fully' | 'partially') Space+)? 'supports' Space+ ([a-z0-9] | '-')+
 
 Version ::= (Digit | '.')+