Per the 20171109 joint TTWG/CSS meeting
As illustrated at linePadding.pdf and specified at https://tech.ebu.ch/docs/tech/tech3380.pdf, a common captioning style requires the background of a line area to be extended at line start and line end. This styling capability is not currently supported by CSS.
See for example regions 3 and 4 at https://codepen.io/palemieux/pen/vyzbqW . Region 1 and 2 illustrate a region with and without line padding applied.
This capability mirrors the ebutts:linePadding styling attribute at https://www.w3.org/TR/ttml-imsc1.0.1/#linepadding , which is demonstrated in the following sample document, which can be rendered at http://sandflow.com/imsc1proc/index.html .
<?xml version="1.0" encoding="UTF-8"?>
<tt xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns="http://www.w3.org/ns/ttml" xmlns:ttp="http://www.w3.org/ns/ttml#parameter"
xmlns:tts="http://www.w3.org/ns/ttml#styling" xmlns:ttm="http://www.w3.org/ns/ttml#metadata"
xmlns:ebutts="urn:ebu:tt:style" xml:lang="en" ttp:timeBase="media" xmlns:ebuttm="urn:ebu:tt:metadata" >
<head>
<metadata>
<ebuttm:documentMetadata>
<ebuttm:conformsToStandard>urn:ebu:tt:distribution:2014-01</ebuttm:conformsToStandard>
<ebuttm:conformsToStandard>http://www.w3.org/ns/ttml/profile/imsc1/text</ebuttm:conformsToStandard>
</ebuttm:documentMetadata>
</metadata>
<styling>
<style xml:id="baseStyle" tts:color="#FFFFFF" tts:textAlign="center" tts:lineHeight="100%"/>
<style xml:id="blackBackground" tts:backgroundColor="#000000"/>
<style xml:id="greenBackground" tts:backgroundColor="#00FF00"/>
<style xml:id="withLinePadding" ebutts:linePadding="0.5c"/>
</styling>
<layout>
<region xml:id="area1" tts:origin="5% 10%" tts:extent="90% 20%" tts:displayAlign="center"/>
<region xml:id="area2" tts:origin="5% 70%" tts:extent="90% 20%" tts:displayAlign="center"/>
</layout>
</head>
<body>
<div style="baseStyle">
<p xml:id="s1" region="area1" begin="00:00:01" end="00:00:09">
<span style="greenBackground">Centered text on two lines<br/>without padding.</span>
</p>
<p xml:id="s2" region="area2" style="withLinePadding" begin="00:00:01" end="00:00:09">
<span style="blackBackground">Centered text on two lines<br/>with padding.</span>
</p>
</div>
</body>
</tt>
Per the 20171109 joint TTWG/CSS meeting
As illustrated at linePadding.pdf and specified at https://tech.ebu.ch/docs/tech/tech3380.pdf, a common captioning style requires the background of a line area to be extended at line start and line end. This styling capability is not currently supported by CSS.
See for example regions 3 and 4 at https://codepen.io/palemieux/pen/vyzbqW . Region 1 and 2 illustrate a region with and without line padding applied.
This capability mirrors the
ebutts:linePaddingstyling attribute at https://www.w3.org/TR/ttml-imsc1.0.1/#linepadding , which is demonstrated in the following sample document, which can be rendered at http://sandflow.com/imsc1proc/index.html .