RB

Description	Defines the base text element within a ruby annotation, representing the main text to which the annotation applies.
Namespace	1.7 2.0
Category	inline

Attributes

Placement

Specifies how the element is placed relative to surrounding content (e.g., block-level or inline flow).

WritingMode

Defines the direction of text flow (e.g., left-to-right, right-to-left, or vertical).

BackgroundColor

Sets the background color for the element’s content area.

BorderColor

Specifies the color of the border around the element.

BorderStyle

Indicates the style of the border (e.g., solid, dashed, dotted).

BorderThickness

Defines the thickness of the border line in user space units (such as points).

Padding

Determines the space between the element’s border (or boundary) and its inner content.

Color

Applies the primary color (fill or stroke) for the text or graphic content.

role

Main indicator of type. This semantic association allows tools to present and support interaction with the object in a manner that is consistent with user expectations about other objects of that type.

Differences

Well tagged PDF:

The 'RB' (Ruby Base) element in Well-Tagged PDF identifies the base text that is being annotated with ruby text.

It should encapsulate the main text to which the ruby annotation applies, and must be clearly nested within the corresponding Ruby element.

PDFUA:

In PDF/UA, the 'RB' element is essential for linking the primary text with its ruby annotation, ensuring that the relationship is clear to assistive technologies.

The 'RB' element must be tagged in conjunction with its corresponding 'RT' element, clearly distinguishing the base text for accessible rendering.