The basic goal is to make the on-line edition match the print edition -- errors and all. We aren't updating spelling or usage or changing it to match modern American usage.
When the page images are available, we can refer to the page images and make corrections with confidence. Sometimes the page images are not available at the CCEL, but they are available at Google Books. In that case you can search for some words on the page and try to find a source of page images.
When page images are not available, the goal is to fix obvious typographical errors but not to make changes that are unlikely to be a result of scanning or typing errors.
The goal is to proof all the pages of each book at least twice. At that point we'll consider the book "proofed." Of course, proofing will continue after that point as readers continue to report typos.