Add more comments for rustdoc
Implement Text mode (except for the hard part)
Give tree builder fail!()s unique messages
Add a helper for iterating over active formatting elements
Implement <a> in InBody
rcdom: Append text to existing nodes when they exist
Escape token descriptions in tree builder debug/error output
print-rcdom: Print parse errors
Show atoms as atom!() invocations
Fix missing state transition
Switch to a slightly more correct adoption agency algorithm
Implement frameset modes
Move InsertionMode into main tree builder module
Factor out iteration over html5lib tests
Remove RcDom root field
Run the html5lib tree construction tests
Prefix test cases with which component they test
Add env vars to control which tests we load

Loading consumes a fair amount of time before the point where the built-in test
infrastructure can filter which tests to run.
Example program output tweaks
Don't drop whitespace eagerly in some tree builder modes

The mode might change before we get to the whitespace.