diff --git a/commonmark/src/main/java/org/commonmark/internal/inline/HtmlInlineParser.java b/commonmark/src/main/java/org/commonmark/internal/inline/HtmlInlineParser.java
index ba992e2f..605901c2 100644
--- a/commonmark/src/main/java/org/commonmark/internal/inline/HtmlInlineParser.java
+++ b/commonmark/src/main/java/org/commonmark/internal/inline/HtmlInlineParser.java
@@ -24,8 +24,6 @@ public class HtmlInlineParser implements InlineContentParser {
             .c('"').c('\'').c('=').c('<').c('>').c('`')
             .build();
 
-    private static final AsciiMatcher declaration = AsciiMatcher.builder().range('A', 'Z').build();
-
     @Override
     public ParsedInline tryParse(InlineParserState inlineParserState) {
         Scanner scanner = inlineParserState.scanner();
@@ -58,7 +56,7 @@ public ParsedInline tryParse(InlineParserState inlineParserState) {
                 if (tryCdata(scanner)) {
                     return htmlInline(start, scanner);
                 }
-            } else if (declaration.matches(c)) {
+            } else if (asciiLetter.matches(c)) {
                 if (tryDeclaration(scanner)) {
                     return htmlInline(start, scanner);
                 }
@@ -187,9 +185,9 @@ private static boolean tryCdata(Scanner scanner) {
     }
 
     private static boolean tryDeclaration(Scanner scanner) {
-        // spec: A declaration consists of the string <!, a name consisting of one or more uppercase ASCII letters,
-        // whitespace, a string of characters not including the character >, and the character >.
-        scanner.match(declaration);
+        // spec: A declaration consists of the string <!, an ASCII letter, zero or more characters not including
+        // the character >, and the character >.
+        scanner.match(asciiLetter);
         if (scanner.whitespace() <= 0) {
             return false;
         }
diff --git a/commonmark/src/test/java/org/commonmark/test/HtmlInlineParserTest.java b/commonmark/src/test/java/org/commonmark/test/HtmlInlineParserTest.java
index 0a406778..0172ca43 100644
--- a/commonmark/src/test/java/org/commonmark/test/HtmlInlineParserTest.java
+++ b/commonmark/src/test/java/org/commonmark/test/HtmlInlineParserTest.java
@@ -23,5 +23,8 @@ public void declaration() {
         assertRendering("inline <!FOO>", "<p>inline &lt;!FOO&gt;</p>\n");
         assertRendering("inline <!FOO >", "<p>inline <!FOO ></p>\n");
         assertRendering("inline <!FOO 'bar'>", "<p>inline <!FOO 'bar'></p>\n");
+
+        // Lowercase
+        assertRendering("inline <!foo bar>", "<p>inline <!foo bar></p>\n");
     }
 }