I think that there might be a security flaw with this method. If the computer which you disconnect from the internet is already compromised it may 'log' the data while it is offline and then send the data once it comes back online. I'm not super techy, but I imagine that someone who is would be able to compromise a computer to function in this way. If so, you could perform the same method but copy the htm file over to a device which has never been on the internet (a dumb computer) and I imagine be pretty safe.
If you are sure that I am wrong then fair play, but if not I'd reconsider your security and alter your post to advise others accordingly. I wouldn't want you or anybody else losing any money.
This is an interesting point. Clearing the cache, browser and cookies wouldn't take care of these potential logs? Hmmm. Okay, looks like I will revise my post with a note about using another device to be extra safe.