[HowlerX]

Originally Posted by SmithM

It works fine when I embed the fonts in epub file.

I'm assuming you are going to be reading this on a Kindle e-ink device or a tablet or phone with the Kindle app installed. Since all the devices and apps mentioned have all 4 fonts you've declared, there is no need to use "font-face" declarations. If you want Kindle devices to force the use of a particular font, this is what I've found works (using your css stylesheet as a starting point):

.baskerville {
	font-family: "Baskerville", serif;
}
.opendyslexic {
	font-family: "OpenDyslexic", serif;
}
.bookerly {
	font-family: "Bookerly", serif;
}
.helvetica {
	font-family: "Helvetica Neue LT", sans-serif;
}

Not sure why you have to use "Helvetica Neue LT" myself. I'm assuming it's because the Kindle App on iOS uses that particular variant of Helvetica. I tested it on your sample converted to KFX and loaded it on my Kindle and it displays as intended. Hope this helps.