LibWeb: Treat CSS at-rule names as case-insensitive

Author: https://github.com/tcl3
Commit: a6efdb1068
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3879
Reviewed-by: https://github.com/gmta ✅

Libraries/LibWeb/CSS/Parser/Tokenizer.cpp

@@ -1227,7 +1227,7 @@ Token Tokenizer::consume_a_token()
         // If the next 3 input code points would start an ident sequence, consume an ident sequence, create
         // an <at-keyword-token> with its value set to the returned value, and return it.
         if (would_start_an_ident_sequence(peek_triplet())) {
-            auto name = consume_an_ident_sequence();
+            auto name = consume_an_ident_sequence().to_ascii_lowercase();
             return create_value_token(Token::Type::AtKeyword, move(name), input_since(start_byte_offset));
         }