TTML 1.0 (Editorial)
TTML 1.0 Test Suite
TTML IMSC 1.0
TTML IMSC 2.0
TTML Simple Delivery Profile for Closed Captions (US)
TTML versions and profiles note
TTML WebVTT mapping note
Frans de Jong
Loretta Guarino Reid
Philippe Le Hégaret
The current rendering rules for TTML allow for a background colour to be specified, to be painted behind rendered text. However the edges of this background area are coincident with the foreground text boundary, or with the entire region, neither of which meets all use cases. A common practice in rendering subtitles against video is to extend slightly the background area painted behind the foreground text, e.g. by the width of one space character .
 https://industry.youview.com/resources/YouView_Core_Technical_Specification_1.0.pdf section 22.214.171.124
No proposed solution is available to address the general case of this, which has some complexity e.g. what to do if a <span> has a different background colour from its parent <p> and the text in the span is wrapped over a line. However the specific use case can be addressed either by specifying a generic rendering rule to cover all text in a region or even a document, e.g. a tts:rowPadding attribute with a length value. Alternative proposals for indicating the requirement for presentation processors to use this rendering behaviour are welcomed!
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Glenn: having some images would help to understand
Owner is Andreas + Nigel
[pal]: CSS folks mentioned box-decoration-break as a possibiliy
Examples, proposed syntax and mapping to HTML/CSS now added to https://www.w3.org/wiki/TTML/changeProposal015#box-decoration-break
[nigel]: (TTWG F2F today) We will support the EBU line padding proposal with the combination of padding on inline content elements and box-decoration-break.
Addressed  by defining that padding on span is cloned at line break boundaries.