--- a/servo/components/style/stylesheets/rule_parser.rs
+++ b/servo/components/style/stylesheets/rule_parser.rs
@@ -511,11 +511,12 @@ impl<'a, 'b, 'i> QualifiedRuleParser<'i>
 
 /// Calculates the location of a rule's source given an offset.
 fn get_location_with_offset(
     location: SourceLocation,
     offset: u64
 ) -> SourceLocation {
     SourceLocation {
         line: location.line + offset as u32,
-        column: location.column,
+        // Column offsets are not yet supported, but Gecko devtools expect 1-based columns.
+        column: location.column + 1,
     }
 }