All of the following documentation is generated on-the-fly, in your browser, by slowparse.js and errors.jquery.js. If something looks amiss, run the actual output against the expected results at the test suite; if errors are reported, please feel free to file a bug.
You can also consult the live demo to see this error reporting used in an interactive widget.
The HTML5 specification actually allows unquoted attribute values, so long as they don't have spaces in them. However, this isn't commonly done outside of minifying HTML. Furthermore, by complicating the standard, our predictions about the user's intent also become ambiguous, which results in more cryptic and confusing error messages. By choosing to enforce monotony, we ease the learning process and provide more helpful errors.
The tree-inspectors.js script must be included to enable these errors.