Considering the date at which you posted this thread, maybe you've figured out how to do this already, but it may be useful for other people having the same question!!
To substitute your string, you don't need to find it using grep, you only need to use sed to find the regexp and substitute it for you, like this:
sed 's/[Criteria]/[Replacementstring]/' [file].html > [newfile].html
Of course, if you want to do this for many files, you'd better do this in a shell script:
for i in foo.html bar.html
sed 's/[Criteria]/[Replacementstring]/' $i > $i.new