Clarify meaning of font* properties being applied to a paragraph.

[skynavga]

The lack of specific text in the specification that explains the intended application of the following properties to a p element has resulted in certain misinterpretations:

• fontFamily
• fontShear
• fontSize
• fontStyle
• fontWeight

The intent of having these apply to p is for the sole purpose of computing the default line height that applies for the purpose of formatting the paragraph. In XSL-FO, this is called the nominal-requested-line-rectangle.

Since the information that describes this usage is common for all five of these properties, we should employ only one description, and refer to it from all five attribute definitions.

Further, it may be necessary to add additional notes for some properties where there has developed a misunderstanding that the property should be applied to the block areas generated by a p rather than (or as well as) being applied to the glyph area descendants of these block areas. For example, the fontShear property is not intended to cause a shear transformation to occur on the block areas, but only on individual glyph area descendants of block areas.

See also #784 and a relevant email thread.

[skynavga]

Note well that we have previously encoded the semantic dependency of line height computation on the relevant font* properties in the algorithm found under tts:lineHeight.

[palemieux]

The lack of specific text in the specification that explains the intended application of the following properties to a p element has resulted in certain misinterpretations:

None of these properties applied to p in TTML1, and it did not cause interoperability issues, so I do not understand why this was changed in TTML2.

[skynavga]

Merged early per WG resolution.