Since everytime you are using Filter.ExtractStyleBlocks as a filter, you need css-tidy.
A mention in the INSTALL.md is nice to have, but nobody in the PHP world reads that and it does not even appear on the https://packagist.org/packages/ezyang/htmlpurifier site.

I would suggest adding a suggest in the composer.json, so people know that they need to install this library when they want to use the above mentioned filter.