I have been wrestling with this plugin for some time now. It does work but there are a few funnies such as the 75% one.
According to the author:
“There’s no way to tell the status of the managed resource store, so you can’t make an accurate progress bar (unless you use a normal resource store and manage it yourself).”
Consequently he put this in the java script:
if(progressBar.step >= 0.75) {
window.clearInterval(timerId);
}
If you remove that from wp-offline.js, guess what, it goes through to 100% and sits there until it actually finishes.
What bugs me more is that everytime you click on the offline icon it has to download everything rather than work out if anything has changed and act accordingly. Any ideas on that?