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.