Add help text for style-flags properties

src/textual/css/_help_text.py

@@ -10,6 +10,7 @@ from textual.css.constants import (
     VALID_EDGE,
     VALID_ALIGN_HORIZONTAL,
     VALID_ALIGN_VERTICAL,
+    VALID_STYLE_FLAGS,
 )
 
 if sys.version_info >= (3, 8):
@@ -384,7 +385,7 @@ def border_property_help_text(
                         ],
                     ),
                     Bullet(
-                        f"Valid values for <bordertype> are:\n  {friendly_list(VALID_BORDER)}"
+                        f"Valid values for <bordertype> are:\n{friendly_list(VALID_BORDER)}"
                     ),
                     Bullet(
                         f"Colors can be specified using hex, RGB, or ANSI color names"
@@ -626,3 +627,36 @@ def offset_single_axis_help_text(property_name: str) -> HelpText:
             Bullet(f"Valid scalar units are {friendly_list(SYMBOL_UNIT)}"),
         ],
     )
+
+
+def style_flags_property_help_text(
+    property_name: str, value: str, context: StylingContext | None
+) -> HelpText:
+    property_name = _contextualize_property_name(property_name, context)
+    return HelpText(
+        summary=f"Invalid value '{value}' in [i]{property_name}[/] property",
+        bullets=[
+            Bullet(
+                f"Style flag values such as [i]{property_name}[/] expect space-separated values"
+            ),
+            Bullet(f"Permitted values are {friendly_list(VALID_STYLE_FLAGS)}"),
+            *ContextSpecificBullets(
+                inline=[
+                    Bullet(
+                        markup="In Python, you can supply a string or Style object",
+                        examples=[
+                            Example(
+                                f'widget.styles.{property_name} = "bold italic underline"'
+                            )
+                        ],
+                    ),
+                ],
+                css=[
+                    Bullet(
+                        markup="In Textual CSS, you can supply style flags separated by spaces",
+                        examples=[Example(f"{property_name}: bold italic underline;")],
+                    )
+                ],
+            ).get_by_context(context),
+        ],
+    )

src/textual/css/_style_properties.py

@@ -19,6 +19,7 @@ from ._help_text import (
     layout_property_help_text,
     fractional_property_help_text,
     offset_property_help_text,
+    style_flags_property_help_text,
 )
 from ._help_text import (
     spacing_wrong_number_of_values,
@@ -28,7 +29,7 @@ from ._help_text import (
 )
 from ..color import Color, ColorPair, ColorParseError
 from ._error_tools import friendly_list
-from .constants import NULL_SPACING
+from .constants import NULL_SPACING, VALID_STYLE_FLAGS
 from .errors import StyleTypeError, StyleValueError
 from .scalar import (
     get_symbols,
@@ -790,20 +791,6 @@ class ColorProperty:
 class StyleFlagsProperty:
     """Descriptor for getting and set style flag properties (e.g. ``bold italic underline``)."""
 
-    _VALID_PROPERTIES = {
-        "none",
-        "not",
-        "bold",
-        "italic",
-        "underline",
-        "overline",
-        "strike",
-        "b",
-        "i",
-        "u",
-        "o",
-    }
-
     def __set_name__(self, owner: Styles, name: str) -> None:
         self.name = name
 
@@ -840,12 +827,14 @@ class StyleFlagsProperty:
                 obj.refresh()
         else:
             words = [word.strip() for word in style_flags.split(" ")]
-            valid_word = self._VALID_PROPERTIES.__contains__
+            valid_word = VALID_STYLE_FLAGS.__contains__
             for word in words:
                 if not valid_word(word):
                     raise StyleValueError(
-                        f"unknown word {word!r} in style flags, "
-                        f"valid values are {friendly_list(self._VALID_PROPERTIES)}"
+                        f"unknown word {word!r} in style flags",
+                        help_text=style_flags_property_help_text(
+                            self.name, word, context="inline"
+                        ),
                     )
             style = Style.parse(style_flags)
             if obj.set_rule(self.name, style):

src/textual/css/_styles_builder.py

@@ -20,6 +20,7 @@ from ._help_text import (
     align_help_text,
     offset_property_help_text,
     offset_single_axis_help_text,
+    style_flags_property_help_text,
 )
 from .constants import (
     VALID_ALIGN_HORIZONTAL,
@@ -30,6 +31,7 @@ from .constants import (
     VALID_DISPLAY,
     VALID_OVERFLOW,
     VALID_VISIBILITY,
+    VALID_STYLE_FLAGS,
 )
 from .errors import DeclarationError, StyleValueError
 from .model import Declaration
@@ -542,6 +544,15 @@ class StylesBuilder:
     process_scrollbar_background_active = process_color
 
     def process_text_style(self, name: str, tokens: list[Token]) -> None:
+        for token in tokens:
+            value = token.value
+            if value not in VALID_STYLE_FLAGS:
+                self.error(
+                    name,
+                    token,
+                    style_flags_property_help_text(name, value, context="css"),
+                )
+
         style_definition = " ".join(token.value for token in tokens)
         self.styles.text_style = style_definition
 

src/textual/css/constants.py

@@ -31,5 +31,18 @@ VALID_BOX_SIZING: Final = {"border-box", "content-box"}
 VALID_OVERFLOW: Final = {"scroll", "hidden", "auto"}
 VALID_ALIGN_HORIZONTAL: Final = {"left", "center", "right"}
 VALID_ALIGN_VERTICAL: Final = {"top", "middle", "bottom"}
+VALID_STYLE_FLAGS: Final = {
+    "none",
+    "not",
+    "bold",
+    "italic",
+    "underline",
+    "overline",
+    "strike",
+    "b",
+    "i",
+    "u",
+    "o",
+}
 
 NULL_SPACING: Final = Spacing.all(0)

src/textual/css/styles.py

@@ -219,7 +219,7 @@ class StylesBase(ABC):
     align_vertical = StringEnumProperty(VALID_ALIGN_VERTICAL, "top")
 
     def __eq__(self, styles: object) -> bool:
-        """Check that Styles containts the same rules."""
+        """Check that Styles contains the same rules."""
         if not isinstance(styles, StylesBase):
             return NotImplemented
         return self.get_rules() == styles.get_rules()

tests/css/test_styles.py

@@ -3,7 +3,7 @@ import pytest
 from rich.style import Style
 
 from textual.color import Color
-from textual.css.errors import StyleTypeError, StyleValueError
+from textual.css.errors import StyleValueError
 from textual.css.styles import Styles, RenderStyles
 from textual.dom import DOMNode