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* Summary of substantive changes *
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NOTE 1: This summary is provided for illustration only. For an exhaustive list of changes, please
refer to the changesets at https://github.com/w3c/ttml1 or use the W3C diff tool at
http://services.w3.org/htmldiff across published versions of the specification.

NOTE 2: Issues and their dispositions are documented at https://github.com/w3c/ttml1/issues

From CR1 (20180424)
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- clarified default value blur radius in tts:textOutline (#351)

- update tts:lineHeight='normal' algorithm to match TTML2 (#354)

From Second Edition (20130924)
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- clarified tts:fontSize semantics (#206, #214 and #215)

- specified timing semantics for <br> elements (#339)

- clarified tts:direction on <p> elements (#324)

- clarified LWSP in color expressions (#322)

- clarified validation of undefined items in TT namespaces (#251)

- clarified anonymous span construction, implicit duration for anonymous spans,
  implicit duration for spans with text nodes exclusively, and implicit duration 
  for <set> (#310, #193)
  
- clarified that LWSPs are permitted in tts:fontFamily (#248)

- added initial value fallback to non-inheritable properties (#200)

- added <br> to elements to which style applies (#249)

- specified application defaults for frame rate multiplier and sub-framerate (#192)

- prevent pruning of <br> elements (#197)

- handled forward interoperability of attribute extensions in TT namespaces (#196)

- prevent <set> elements from being pruned in ISD construction (#216)

- clarified that the inherited value of tts:lineHeight='normal' is 'normal' (#220)

- clarified application of tts:visibility to child elements (#207)

- clarified tts:textDecoration inheritance (#219)

- clarified cell metrics for tts:textOutline (#218)

- clarified tts:zIndex stacking context (#241)