Need to delete first N lines of grep result files
What's wrong with iterating over the files directly¹?
And you might want to add the -i flat to sed so that files are edited in-place
grep -r -l "//360cdn.win/c.css" | while read fdo sed -e '1,9d' -i "${f}"done¹ well, you might get problems if your files contain newlines and the like.but then...if your website contains files with newlines, you probably have other problems anyhow...