Improve Rhapsode's build with newer tools, fix crashes in history infrastructure.

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.