{"action":"created","issue":{"url":"https://api.github.com/repos/w3c/ttml1/issues/220","repository_url":"https://api.github.com/repos/w3c/ttml1","labels_url":"https://api.github.com/repos/w3c/ttml1/issues/220/labels{/name}","comments_url":"https://api.github.com/repos/w3c/ttml1/issues/220/comments","events_url":"https://api.github.com/repos/w3c/ttml1/issues/220/events","html_url":"https://github.com/w3c/ttml1/issues/220","id":188938130,"number":220,"title":"lineHeight \"computed\" inheritance","user":{"login":"palemieux","id":4871350,"avatar_url":"https://avatars.githubusercontent.com/u/4871350?v=3","gravatar_id":"","url":"https://api.github.com/users/palemieux","html_url":"https://github.com/palemieux","followers_url":"https://api.github.com/users/palemieux/followers","following_url":"https://api.github.com/users/palemieux/following{/other_user}","gists_url":"https://api.github.com/users/palemieux/gists{/gist_id}","starred_url":"https://api.github.com/users/palemieux/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/palemieux/subscriptions","organizations_url":"https://api.github.com/users/palemieux/orgs","repos_url":"https://api.github.com/users/palemieux/repos","events_url":"https://api.github.com/users/palemieux/events{/privacy}","received_events_url":"https://api.github.com/users/palemieux/received_events","type":"User","site_admin":false},"labels":[{"id":279479775,"url":"https://api.github.com/repos/w3c/ttml1/labels/enhancement","name":"enhancement","color":"84b6eb","default":true}],"state":"open","locked":false,"assignee":null,"assignees":[],"milestone":null,"comments":24,"created_at":"2016-11-12T22:18:16Z","updated_at":"2016-11-15T22:13:19Z","closed_at":null,"body":"Section 8.4.4.2(6)(b) specifies that `lineHeight` is set to its initial value of `\"normal\"` on `<region>`.\r\n\r\nSection 8.4.4.2(6)(c) specifies that `<body>` will inherit the _computed value_ of `lineHeight` from its parent `<region>`.\r\n\r\nThe computed value of `lineHeight` on `<region>` is therefore, say, 125% of the computed value of `fontSize` on `<region>`, so `125/15rh`.\r\n\r\nSo the computed values of `lineHeight` and `fontSize` on `<body>` are `125/15rh` and `400/15rh`, respectively.\r\n\r\nThese computed values are then propagated down by inheritance to the `<p>`, where \"Hello\" and \"Bye\" will overlap since the font size is significantly larger than the line height.\r\n\r\nIs this the intended outcome and correct interpretation, or alternatively should initial values only be applied to elements to which the styles applies or does \"normal\" have special inheritance rules?\r\n\r\n```\r\n<?xml version=\"1.0\" encoding=\"UTF-8\"?>\r\n<tt xmlns=\"http://www.w3.org/ns/ttml\" xmlns:tts=\"http://www.w3.org/ns/ttml#styling\">\r\n    <head>\r\n        <layout>\r\n            <region xml:id=\"bottom\" tts:origin=\"10% 10%\" tts:extent=\"80% 80%\" />\r\n        </layout>\r\n    </head>\r\n    <body tts:fontSize=\"4c\">\r\n        <div>\r\n            <p region=\"bottom\" begin=\"00:00:10.000\" end=\"00:00:20.000\">Hello<br/>Bye</p>\r\n        </div>\r\n    </body>\r\n</tt>\r\n```\r\n\r\nNOTE: `100rh` equals the computed height of root container."},"comment":{"url":"https://api.github.com/repos/w3c/ttml1/issues/comments/260786958","html_url":"https://github.com/w3c/ttml1/issues/220#issuecomment-260786958","issue_url":"https://api.github.com/repos/w3c/ttml1/issues/220","id":260786958,"user":{"login":"nigelmegitt","id":4988915,"avatar_url":"https://avatars.githubusercontent.com/u/4988915?v=3","gravatar_id":"","url":"https://api.github.com/users/nigelmegitt","html_url":"https://github.com/nigelmegitt","followers_url":"https://api.github.com/users/nigelmegitt/followers","following_url":"https://api.github.com/users/nigelmegitt/following{/other_user}","gists_url":"https://api.github.com/users/nigelmegitt/gists{/gist_id}","starred_url":"https://api.github.com/users/nigelmegitt/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/nigelmegitt/subscriptions","organizations_url":"https://api.github.com/users/nigelmegitt/orgs","repos_url":"https://api.github.com/users/nigelmegitt/repos","events_url":"https://api.github.com/users/nigelmegitt/events{/privacy}","received_events_url":"https://api.github.com/users/nigelmegitt/received_events","type":"User","site_admin":false},"created_at":"2016-11-15T22:13:19Z","updated_at":"2016-11-15T22:13:19Z","body":"The way I would describe this, the value `normal` is itself inheritable as a computed value, which is only evaluated into a real size when stacking the line areas associated with the `p` element to which it applies.\r\n\r\nIn terms of actions, at the very least, we need an erratum to TTML1SE explaining that the computed value associated with the specified value of \"normal\" is \"normal\".\r\n\r\nRe-reading the text:\r\n\r\n> If the value of this attribute is normal, then the computed value of the style property must be considered to be no less than the size of the largest font size that applies to the element and its descendant elements in the intermediate synchronic document as determined by 9.3.2 Intermediate Synchronic Document Construction.\r\n\r\nI see that this does not actually preclude using the largest font size of all descendants; it may be worth adding by errata an informative note to explain this and that it can be used to avoid baselines jumping at ISD junctions, which can still occur if a span is introduced with larger-sized text, since \"normal\" is presumably only evaluated on the `p` element to which `tts:lineHeight` applies. For TTML2 I would prefer to see an even more clearly specified rule however, while still retaining the ability to size based on the largest font size of the `p`'s contents.\r\n\r\nIn summary, I think there are two actions for TTML1, both in an erratum:\r\n\r\n* [ ] Clarify that the computed value of `normal` is `normal` up until it has to be evaluated to an actual size, on the `p` element.\r\n* [ ] Informatively note that the largest font size of all descendants of the `p` can be used, to avoid baseline jumping.\r\n\r\nAlso for TTML2:\r\n\r\n* [ ] Capture these changes and any further implications for TTML2 as issues."}}
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