↑ ↑ ↑ ✔ (mostly

I have MY fav styling. Some were because my first reader had Adobe block numbers on the right so I made margin-right: 16px)
I start with the Debug in the Calibre editor. There are some 'errors' I ignore (eg Webkit, more than 1 CSS occurrence (I do check, but that is really legal)
Syntax errors are always to be fixed as some devices ignore a CSS with those. Practice

Books by Vellum all PASS, but have those huge chapter headings (i have saved searches to find and reduce (or comment out the 12%Minimum)
I comment out, rather than totally hide my CSS adjustments YMMV
It will take some time to get fast at fixing or just plain touchup. Saved searches are handy. Some fixes take me 30 seconds to mash it my way. Some take

because the selfpub had no clue what a clean coded book can look like (eg seperate (and different) stylesheet per chapter)