diff --git a/.changeset/fix-html-parser-digit-tag-start.md b/.changeset/fix-html-parser-digit-tag-start.md
new file mode 100644
index 000000000000..7c97f6df2819
--- /dev/null
+++ b/.changeset/fix-html-parser-digit-tag-start.md
@@ -0,0 +1,5 @@
+---
+"@biomejs/biome": patch
+---
+
+Fixed [#8363](https://github.com/biomejs/biome/issues/8363): HTML parser no longer crashes when encountering a `<` character followed by a digit in text content (e.g., `<12 months`). The parser now correctly emits an "Unescaped `<` bracket character" error instead of treating `<12` as a tag name and crashing.
diff --git a/crates/biome_html_parser/src/lexer/mod.rs b/crates/biome_html_parser/src/lexer/mod.rs
index 8c6dca5fd6e3..3f108b4e883c 100644
--- a/crates/biome_html_parser/src/lexer/mod.rs
+++ b/crates/biome_html_parser/src/lexer/mod.rs
@@ -219,7 +219,7 @@ impl<'src> HtmlLexer<'src> {
                 // https://html.spec.whatwg.org/multipage/syntax.html#start-tags
                 if self
                     .peek_byte()
-                    .is_some_and(|b| is_tag_name_byte(b) || b == b'!' || b == b'/' || b == b'>')
+                    .is_some_and(|b| is_tag_start_byte(b) || b == b'!' || b == b'/' || b == b'>')
                 {
                     self.consume_l_angle()
                 } else {
@@ -1238,6 +1238,12 @@ fn is_tag_name_byte(byte: u8) -> bool {
     byte.is_ascii_alphanumeric() || byte == b'-' || byte == b':' || byte == b'.'
 }
 
+fn is_tag_start_byte(byte: u8) -> bool {
+    // Tag names must start with an ASCII letter (not a digit)
+    // https://html.spec.whatwg.org/#valid-custom-element-name
+    byte.is_ascii_alphabetic()
+}
+
 fn is_attribute_name_byte(byte: u8) -> bool {
     // https://html.spec.whatwg.org/#attributes-2
     byte.is_ascii()
diff --git a/crates/biome_html_parser/tests/html_specs/error/element/tag-name-starts-with-digit.html b/crates/biome_html_parser/tests/html_specs/error/element/tag-name-starts-with-digit.html
new file mode 100644
index 000000000000..9fdb041715bf
--- /dev/null
+++ b/crates/biome_html_parser/tests/html_specs/error/element/tag-name-starts-with-digit.html
@@ -0,0 +1 @@
+<div>Target <12 months</div>
diff --git a/crates/biome_html_parser/tests/html_specs/error/element/tag-name-starts-with-digit.html.snap b/crates/biome_html_parser/tests/html_specs/error/element/tag-name-starts-with-digit.html.snap
new file mode 100644
index 000000000000..24b93ce85cce
--- /dev/null
+++ b/crates/biome_html_parser/tests/html_specs/error/element/tag-name-starts-with-digit.html.snap
@@ -0,0 +1,100 @@
+---
+source: crates/biome_html_parser/tests/spec_test.rs
+expression: snapshot
+---
+## Input
+
+```html
+<div>Target <12 months</div>
+
+```
+
+
+## AST
+
+```
+HtmlRoot {
+    bom_token: missing (optional),
+    frontmatter: missing (optional),
+    directive: missing (optional),
+    html: HtmlElementList [
+        HtmlElement {
+            opening_element: HtmlOpeningElement {
+                l_angle_token: L_ANGLE@0..1 "<" [] [],
+                name: HtmlTagName {
+                    value_token: HTML_LITERAL@1..4 "div" [] [],
+                },
+                attributes: HtmlAttributeList [],
+                r_angle_token: R_ANGLE@4..5 ">" [] [],
+            },
+            children: HtmlElementList [
+                HtmlContent {
+                    value_token: HTML_LITERAL@5..12 "Target" [] [Whitespace(" ")],
+                },
+                HtmlContent {
+                    value_token: HTML_LITERAL@12..13 "<" [] [],
+                },
+                HtmlContent {
+                    value_token: HTML_LITERAL@13..22 "12 months" [] [],
+                },
+            ],
+            closing_element: HtmlClosingElement {
+                l_angle_token: L_ANGLE@22..23 "<" [] [],
+                slash_token: SLASH@23..24 "/" [] [],
+                name: HtmlTagName {
+                    value_token: HTML_LITERAL@24..27 "div" [] [],
+                },
+                r_angle_token: R_ANGLE@27..28 ">" [] [],
+            },
+        },
+    ],
+    eof_token: EOF@28..29 "" [Newline("\n")] [],
+}
+```
+
+## CST
+
+```
+0: HTML_ROOT@0..29
+  0: (empty)
+  1: (empty)
+  2: (empty)
+  3: HTML_ELEMENT_LIST@0..28
+    0: HTML_ELEMENT@0..28
+      0: HTML_OPENING_ELEMENT@0..5
+        0: L_ANGLE@0..1 "<" [] []
+        1: HTML_TAG_NAME@1..4
+          0: HTML_LITERAL@1..4 "div" [] []
+        2: HTML_ATTRIBUTE_LIST@4..4
+        3: R_ANGLE@4..5 ">" [] []
+      1: HTML_ELEMENT_LIST@5..22
+        0: HTML_CONTENT@5..12
+          0: HTML_LITERAL@5..12 "Target" [] [Whitespace(" ")]
+        1: HTML_CONTENT@12..13
+          0: HTML_LITERAL@12..13 "<" [] []
+        2: HTML_CONTENT@13..22
+          0: HTML_LITERAL@13..22 "12 months" [] []
+      2: HTML_CLOSING_ELEMENT@22..28
+        0: L_ANGLE@22..23 "<" [] []
+        1: SLASH@23..24 "/" [] []
+        2: HTML_TAG_NAME@24..27
+          0: HTML_LITERAL@24..27 "div" [] []
+        3: R_ANGLE@27..28 ">" [] []
+  4: EOF@28..29 "" [Newline("\n")] []
+
+```
+
+## Diagnostics
+
+```
+tag-name-starts-with-digit.html:1:13 parse ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  × Unescaped `<` bracket character. Expected a tag or escaped character.
+  
+  > 1 │ <div>Target <12 months</div>
+      │             ^
+    2 │ 
+  
+  i Replace this character with `&lt;` to escape it.
+  
+```