More lenient markup

Author: willmcgugan

CHANGELOG.md

@@ -17,6 +17,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Added experimental opt-in support for https://github.com/willmcgugan/textual-speedups
 
+### Changed
+
+- Content markup is now more lenient; if a 'tag' doesn't contain a valid style it will be included verbatim.
+
 ## [3.3.0] - 2025-06-01
 
 ### Fixed

src/textual/markup.py

@@ -391,10 +391,20 @@ def process_text(template_text: str, /) -> str:
                 text_append(text_content)
                 position += len(text_content)
             else:
-                opening_tag = "".join(tag_text).strip()
-                style_stack.append(
-                    (position, opening_tag, normalize_markup_tag(opening_tag))
-                )
+                opening_tag = "".join(tag_text)
+
+                if not opening_tag.strip():
+                    blank_tag = f"[{opening_tag}]"
+                    text_append(blank_tag)
+                    position += len(blank_tag)
+                else:
+                    style_stack.append(
+                        (
+                            position,
+                            opening_tag,
+                            normalize_markup_tag(opening_tag.strip()),
+                        )
+                    )
 
         elif token_name == "open_closing_tag":
             tag_text = []

tests/test_markup.py

@@ -11,6 +11,10 @@
     [
         ("", Content("")),
         ("[", Content("[")),
+        ("[]", Content("[]")),
+        ("[ ", Content("[ ")),
+        ("[  ", Content("[  ")),
+        ("[  ]", Content("[  ]")),
         ("[0", Content("[0")),
         ("[0]", Content("[0]")),
         ("[red", Content("[red")),