diff --git a/src/attr.rs b/src/attr.rs
index 08ce9f0..32c72bf 100644
--- a/src/attr.rs
+++ b/src/attr.rs
@@ -262,11 +262,11 @@ impl<I: Iterator<Item = char>> Parser<I> {
     }
 }
 
-fn is_name_start(c: char) -> bool {
+pub fn is_name_start(c: char) -> bool {
     c.is_ascii_alphanumeric() || matches!(c, '_' | ':')
 }
 
-fn is_name(c: char) -> bool {
+pub fn is_name(c: char) -> bool {
     is_name_start(c) || c.is_ascii_digit() || matches!(c, '-')
 }
 
diff --git a/src/block.rs b/src/block.rs
index 68279cf..4b89599 100644
--- a/src/block.rs
+++ b/src/block.rs
@@ -654,8 +654,12 @@ impl IdentifiedBlock {
             f @ ('`' | ':' | '~') => {
                 let fence_length = 1 + (&mut chars).take_while(|c| *c == f).count();
                 let spec = &line_t[fence_length..].trim_start();
-                let valid_spec =
-                    !spec.chars().any(char::is_whitespace) && !spec.chars().any(|c| c == '`');
+                let valid_spec = if f == ':' && !spec.starts_with('=') {
+                    spec.chars().next().map_or(true, attr::is_name_start)
+                        && spec.chars().skip(1).all(attr::is_name)
+                } else {
+                    !spec.chars().any(char::is_whitespace) && !spec.chars().any(|c| c == '`')
+                };
                 let skip = line_t.len() - spec.len();
                 (valid_spec && fence_length >= 3).then(|| {
                     (