As an experiment, I ran an html book through html tidy online. I got one error and 58 "warnings." The Tidy output had incorporated all the warnings.
I then fixed the one error in the original html file and opened it in Sigil. I saved it as filename.epub and clicked on the Tidy icon. (I am using 5.3.) The "validation results" window at the bottom of the page remained blank.
1) Am I correct in assuming that those 58 warnings have all been tidied up without my knowing about it?
2) Is there a chance that Tidy could cause a problem with all that dusting and sweeping behind the scenes?