Baldur Bjarnason has turned up a cautionary tale in how not to mistreat readers. It's a trick that is fascinating on a technical level but in practice it is hostile to the reader.
You can see the trick at right. It's the top 2 lines of text.
Friars Lane Digital Services has figured out a way to nag the reader about enabling the publisher recommended fonts on the Kindle. The nag message shown at the top of the image is written in a special font that replaces all of the characters with the space character.
If you can see the warning message then the publisher recommended fonts are not enabled. Looking at it strictly as a technical trick, it's pretty cool.
While I appreciate it on a technical level, let me also say that if anyone copies the trick mentioned in this post and uses it in an ebook I will confiscate your ereader. I will also tell everyone that you have developed your own font that combines design elements of Papyrus and comic sans.
So don't use it.
Baldur looked at this trick and wrote a post about respecting the reader. He argues that readers who choose not to use the specific settings picked by the author or publisher should be allowed to make that decision without harassment.
I do agree with him but there's also a more important issue here.
Consider for a moment the people who need a larger font size, or a specific font choice. There are any number of readers with mild visual impairments who have adopted ebooks because they no longer have to depend on large print books, services like Bookshare, or custom reading equipment. And with the aging of the US population, that group of readers is growing at a prodigious rate.
That said, I think anyone who uses this trick misunderstands their relationship to the reader. I'm sure a lot of authors would like to see themselves as a chef who prepared a gourmet dish to be enjoyed by the customer.
Go ahead and think that, but also know this. Only the most vain chef, one enthroned in their own restaurant, would have the gall to tell customers that they are applying the wrong seasoning.
Don't be that guy. I know most wouldn't like being told that by a chef, and similarly readers don't like being nagged by obsessive authors and publishers.