Add a note about writing mode

files/en-us/glossary/block-level_content/index.md

@@ -6,9 +6,11 @@ page-type: glossary-definition
 
 In CSS, content that participates in block layout is called block-level content.
 
-In a block layout, boxes are laid out one after the other, vertically, beginning at the top of a containing block. Each box's left outer edge touches the left edge of the containing block (for right-to-left formatting, right edges touch).\
+In a block layout, boxes are laid out one after the other, vertically, beginning at the top of a containing block. Each box's left outer edge touches the left edge of the containing block.\
 A block-level element always starts on a new line. And it occupies the entire horizontal space of its parent element (container), and vertical space equal to the height of its contents, thereby creating a "block".
 
+> **Note:** The above behaviour of block layout changes if the containing block's [`writing-mode`](/en-US/docs/Web/CSS/writing-mode) is set to value other than [the default value](/en-US/docs/Web/CSS/writing-mode#formal_definition).
+
 > **Note:** HTML (_HyperText Markup Language_) elements historically were categorized as either "block-level" elements or "inline-level" elements. Since this is a presentational characteristic it is nowadays specified by CSS.
 
 ## Examples
@@ -40,3 +42,4 @@ p {
 - [Inline-level content](/en-US/docs/Glossary/Inline-level_content)
 - [Block formatting context](/en-US/docs/Web/Guide/CSS/Block_formatting_context)
 - {{cssxref("display")}}
+- [`writing-mode`](/en-US/docs/Web/CSS/writing-mode)