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RE: Timed tracks

From: Sean Hayes <Sean.Hayes@microsoft.com>
Date: Sat, 8 May 2010 12:26:38 +0000
To: "robert@ocallahan.org" <robert@ocallahan.org>
CC: Philip Jägenstedt <philipj@opera.com>, "public-html@w3.org" <public-html@w3.org>
Message-ID: <8DEFC0D8B72E054E97DC307774FE4B911A5264A7@DB3EX14MBXC315.europe.corp.microsoft.com>
OK, so in order to discuss that we need to get into some details.

The key properties referenced in TTML that XSL 1.1 references from CSS2 with modification, or that XSL 1.1 defines directly and are now being developed in CSS3 are the following:

width, height, left and top, direction,  display-align (currently called alignment in CSS3), text-align, text-decoration, wrap-option and writing-mode.

These all need modification to support a generalised text layout model, the CSS WG has recognised this and is working on it. It is my belief these will not get to last call in CSS3 without being consistent with XSL:FO since the XSL WG is working in conjunction with CSS, but I shall raise a formal objection on behalf of the TTWG if this is the case. Furthermore I would expect XSL:FO to update its references to CSS3 as appropriate anyway so an indirection will not cause an issue, but as CSS3 proceeds to rec status, TTWG will review updating the references to the latest versions as they become available.

The other properties that are referenced in TTML via XSL (font-family, font-size, font-weight, line-height, overflow, padding, unicode-bidi, visibility, z-index) are actually just indirections to CSS2.1 unchanged, display is already referenced directly.

The remaining properties that TTML uses were defined in advance of a stable CSS3 solution and are not in XSL. These are rgba values & opacity and text-outline. Again I see no reason that these would change fundamentally in CSS3, indeed text-outline is included in CSS3 as it is, largely at the request of the TTWG, TTML references can be added to these as they become stable.

It is difficult in the W3C to keep a bunch of related and interdependent specs progressing in different WGs at different speeds in synch; but let’s be clear TTML *is* defined in terms of CSS by reference. So yes, implementing TTML using a CSS engine *is* a good long term solution if you have one that is consistent with CSS3 and will remain so, however TTML is defined the way it is so that a full CSS implementation is not a requirement while retaining presentational interoperability, and so that it is based on stable references; since doing otherwise would limit its usefulness in other contexts.  Defining a new format in the W3C would simply re-encounter all the same logistical problems that TTML has had to negotiate.

From: rocallahan@gmail.com [mailto:rocallahan@gmail.com] On Behalf Of Robert O'Callahan
Sent: Saturday, May 08, 2010 12:21 PM
To: Sean Hayes
Cc: Philip Jägenstedt; public-html@w3.org
Subject: Re: Timed tracks

On Sat, May 8, 2010 at 10:24 PM, Sean Hayes <Sean.Hayes@microsoft.com<mailto:Sean.Hayes@microsoft.com>> wrote:
To quote Maciej " Standards experts will be *extremely* picky about whether the layout behaviour in a browser precisely matches the spec, ... There isn't really room to fudge it." That doesn't jibe very well with " the status of the specs matters very little, what matters is that it is already implemented and shipped in browsers ".  So which is it?

They're both true. Let me try to clarify:
1) implementing TTML using a CSS layout engine isn't an a good long term solution unless TTML's behaviour precisely matches CSS behaviour (and we have a foolproof process in place to ensure that remains true forever*).
2) the maturity of a spec is better judged by comparing its implementation in browsers than by looking at the W3C's label.

* The only really effective way to do that would be to define TTML in terms of CSS by reference.
If the CSS specs change, as seems likely from a WD status, then those implementations will not be compliant.

Properties already widely and interoperably implemented are unlikely to change. Many CSS2.1 and CSS3 properties already fall into that category.

However, be that as it may, if as you say the CSS3 text and box specs are widely implemented, and the same  in Opera and Gecko, WebKit and IE, then you already have the tools to implement TTML - so all is goodness.

Only if TTML is defined in terms of CSS.
"He was pierced for our transgressions, he was crushed for our iniquities; the punishment that brought us peace was upon him, and by his wounds we are healed. We all, like sheep, have gone astray, each of us has turned to his own way; and the LORD has laid on him the iniquity of us all." [Isaiah 53:5-6]
Received on Saturday, 8 May 2010 12:27:12 UTC

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