BBC Subtitle Guidelines technical requirements for EBU-TT-D

The BBC Subtitle Guidelines specify both editorial and technical requirements for hard of hearing subtitles (aka captions) and the files that express them.

Here are the requirements for the validator when validating EBU-TT-D subtitle documents against the (mostly) technical requirements of the guidelines:

ID	Check	Other bits	Action if found	Implemented in ttml-validator?
PP.1	Presence of null bytes	Not permitted in XML	ERROR, remove null bytes and continue
PP.2	Presence of bad encoding	Hard to be certain about, certain strings are likely giveaways. Not sure if this should be done based on text content only or on the entire document pre-parsing	ERROR, re-encode, check again and continue
PP.3	Presence of a BOM	Not permitted	ERROR, remove BOM and continue
X.1	Conforms to EBU-TT-D XSD	If so, can use EBU-TT Live Interop Toolkit to create a domain model for validation. Otherwise, need to use a different approach	ERROR, continue
X.2	Presence of duplicate xml:id values	Not permitted in XML	ERROR, continue
X.3	Presence of unqualified id attributes	Ignored and not specified in TTML	WARN, continue
T.1	tt namespace	Good value: http://www.w3.org/ns/ttml	ERROR, continue assuming that whatever namespace is present is used as the basis for the namespaces of all the other TT elements and attributes that have a namespace
T.2.0	tt attributes	xml:lang present and not empty	ERROR
T.2.1		ttp:timeBase="media" present: requirement	ERROR
T.2.2		ittp:activeArea present: nice to have, not an error if absent	INFO
T.2.3		ttp:cellResolution present: if absent use default "32 15"	INFO
T.3.0	presence of tt/head	If absent, this is an Error; skip tests that depend on it.	ERROR, continue to body tests
T.3.1	presence of tt/head/ttm:copyright	Nice to have, not an error if absent	WARN
T.3.2	presence of tt/head/styling	If absent, this is an Error; skip tests that depend on it	ERROR, continue to inline style tests
T.3.3	presence of tt/head/styling/style elements	If absent, this is an Error; skip tests that depend on it	ERROR, continue to inline style tests
T.3.4	presence of unreferenced style elements	Shouldn't be any	WARN
T.3.5	presence of style elements without an xml:id	Shouldn't be any	ERROR, continue
T.3.6	presence of style elements without any style attributes	Shouldn't be any	WARN
T.3.7	syntactically invalid styling attributes on style element	Shouldn't be any	ERROR, continue
T.3.8	cyclic style attribute references in style elements	Shouldn't be any	ERROR, continue
T.4	style attributes on span that don't apply to span i.e. tts:lineHeight , tts:textAlign , itts:fillLineGap , ebutts:linePadding , tts:extent , tts:origin , tts:displayAlign	Authoring error, rendering will likely be different to expectation	ERROR, continue
T.5	presence of inline style attributes	not permitted in EBU-TT-D	ERROR, continue
T.6	presence of tts:backgroundColor that is non-transparent on elements other than span	Fails BBC requirement	ERROR, continue
T.6.1	span elements where tts:color is too similar to tts:backgroundColor	Error - but we don't have a test for this, and we would have to check it against the nearest ancestor that set a backgroundColor, not just the current element.	In principle, ERROR, continue, In practice, don't do this until we have a methodology
T.7	use of tts:fontFamily with wrong values	BBC requirement ReithSans, Arial, Roboto, proportionalSansSerif, default	ERROR, continue
T.8	computed tts:fontSize on span	BBC requirement around 6.667% of the render area height for landscape video, or 3.75% for vertical. See also T.30.3	INFO for computed values, ERROR if less than 2% or more than 8% or badly formed (e.g. number omitting unit), continue
T.9	computed tts:lineHeight on p	BBC requirement 120-125%	INFO for computed values, ERROR if less than 100% or more than 130%, WARN if normal	(tests the canonical rh value rather than the %age, different for horizontal and vertical video)
T.10	computed tts:textAlign on p	No check, any value is potentially fine	ERROR if invalid value
T.11	ebutts:multiRowAlign	Not expecting, but not necessarily an error. Warn if different to tts:textAlign	INFO if present, WARN if unexpected value
T.12	ebutts:linePadding	If absent, or out of range, fails requirement. Expect in range 0.3c to 0.8c.	ERROR if absent or out of range
T.13	tts:color	Check it is equivalent fully opaque value from permitted list - requirement (white, yellow, cyan or green). Must be correct format.	ERROR if absent or incorrect value
T.14	itts:fillLineGap	Error if not true on p element	ERROR if absent or incorrect value
T.15	Italics	If tts:fontStyle="italic", on any span , warn. BBC requirement.	WARN if present
T.16.0	presence of tt/head/layout	If absent, this is an Error; skip tests that depend on it	ERROR, continue to body tests assuming default region
T.16.1	presence of tt/head/layout/region elements	If absent, this is an Error; skip tests that depend on it	ERROR, continue to body tests assuming default region
T.16.2	presence of unreferenced region elements	Shouldn't be any	WARN
T.16.3	presence of region elements without an xml:id	Shouldn't be any	ERROR, continue
T.17.0	region element without a tts:origin attribute	If absent, this is an Error	ERROR, continue
T.17.1	region element with a badly formed tts:origin attribute	For example using negative values or wrong units	ERROR, continue
T.17.2	region element without a tts:extent attribute	If absent, this is an Error	ERROR, continue
T.17.3	region element with a badly formed tts:extent attribute	For example using negative values or wrong units	ERROR, continue
T.17.4	region element without a (valid) tts:displayAlign attribute	BBC requirement.	ERROR, continue
T.17.5	region element without a tts:overflow attribute set to visible	BBC requirement.	ERROR, continue
T.17.6	region element with a tts:backgroundColor that is set and non-transparent	BBC requirement	ERROR, continue
T.17.7	region element with a tts:wrapOption that is set to wrap	BBC requirement	WARN
T.17.8	p element that is not in any region	Error if occurs	ERROR, continue
T.17.8.1	p element that is pruned by association with a region different to an ancestor element's region	Warning	WARN
T.17.9	region element that goes outside the expected "safe" area	Error if occurs in a referenced region, Warning otherwise. BBC requirement - to deal with both horizontal and vertical modes, without knowing which, no region should have a top/left edge at less than 5% or a bottom/right edge at more than 95%. Ideally, set different parameters for vertical videos compared to others. See also T.30.3	ERROR/WARN, continue
T.18.0	presence of tt/body	Error if absent	ERROR, skip to T.28
T.18.1	presence of tt/body/div	Error if absent. BBC requirement.	ERROR, skip to T.28
T.18.2	presence of tt/body/div/div	Error if present.	ERROR, continue
T.18.3	presence of p child of every div	Error if absent. BBC requirement.	ERROR, continue
T.19	presence of begin , end or dur attributes on body or div	Error if present	ERROR, continue (include in computation of all descendant element times)
T.20	presence of xml:id on every p element	Error if absent. BBC requirement.	ERROR, continue
T.21	presence of character content children of p	Error if present. BBC requirement.	ERROR, continue
T.21.1	presence of new line characters in character content, and absence of <br> elements	Warn if present - suggests that author erroneously thought that new line characters would be interpreted as new lines rather than white space.	WARN. continue
T.21.2	presence of character codes outside the permitted set	Warn if present - browser might fall back and present something, but it might look bad e.g. instead of ♫ U+266B we have seen a file that uses U+1F3B5 which is more of an emoji style note, not present in Reith Sans, which renders badly on black backgrounds	WARN. continue
T.22	presence of begin or end attributes on both a p and a descendant span	Error if present.	ERROR, continue
T.22.1	presence of badly formed time expressions e.g. with frames components etc.	Error if present.	ERROR, continue
T.22.2	presence of p with no span children	Error if present.	ERROR, continue
T.23	presence of nested span elements	Error if present.	ERROR, continue
T.24	Fewer than 2 distinct computed begin times of p elements earlier than 23 minutes.	Error. NB for p elements without times, where their descendant span elements do have times, float the earliest child begin time and the latest child end time to the p for checking this constraint.	ERROR, continue
T.24.1	For segment subtitles if there is any content check it overlaps with the segment time window.	Error if there is content that falls outside the segment window - suggests that the content is mis-timed or the segmentation window is wrong. Note that a flag is needed say if the input file is media-time-relative or segment-time-relative.	ERROR, continue
T.24.2	regions that overlap and are associated with content elements (p primarily) that overlap in time	Error if present.	ERROR, continue
T.25	Lines that are likely to be wholly or partially wrapped off the edge of the viewing area.	Error if present. To compute this, if valid IMSC, can use imscJS to render PNGs and for each ISD check for non-empty pixels at the boundary edges. Otherwise, analyse regions and number of lines selected into each, and use computed tts:displayAlign edge to work out vertical alignment edge and direction, and see if it's likely that any content goes into outermost 5% of rendering area.	ERROR, continue	A bit hard, not done yet
T.26	Short temporal gaps between subtitles	Error if gaps shorter than 0.8s present. BBC requirement. Warning if gaps between 0.8s and 1.5s present.	ERROR or WARNING, continue
T.27	No sibling elements after body	Error	ERROR, continue	Already check for sibling body elements, sibling elements in other namespaces are ok and will be ignored.
T.28	No sibling elements after root tt	Error	ERROR, stop	Happens as part of XML parsing - ElementTree complains of "junk after document element"
T.29	Fails IMSC-HRM	Warning - not yet a formal requirement, but a good sign that there's a problem. OK to implement this check only if the document passes EBU-TT-D XSD validation.	WARN, continue	To do: one method would be to bring in the EBU-TT Live Interoperability Toolkit; would add a lot of size to the project! Another is to bring in the implementation with some rewrites, or even the one from ttconv+imscHRM
T.30.1	Check metadata: documentOriginatingSystem	Should be present, not required.	WARN, continue
T.30.2	Check metadata: conformsToStandard	Should be present and claim conformance to EBU-TT-D 1.0.1 and IMSC 1.0.1 Text	WARN, continue
T.30.3	Check metadata: documentTargetAspectRatio	If present, check against expectation (command line argument?) and also use to set tighter limits on fontSize (T.8) and safe areas (T.17.9).	WARN, continue