Don't let marks interfer with silence collapsing.
Fix markers misserialization.
Propagate local attrs through to output.
Catch more cases of consecutive breaks.
Collapse consecutive breaks again. FIXME: Collapse when one <break> is inside another tag.
Implement new -rhaps-marker property for controlling user interaction.
Extensively refactor to use the CSS engine's counters implementation.
Parse content & counter properties, cleanup property handling.
Integrate CSS Speech Box Model, improve user agent stylesheet.
Draft code for laying out CSS Speech Box Model.