[Turtle91]

Yes, he could just use 2 css sheets.

I think JSWolfe's understanding of CSS is tainted by his experience with ADE. He thinks that the use of anything not understood/supported will cause the entire css sheet to be ignored, instead of just that portion that is not understood/supported. That is a valid concern under ADE but it is not a CSS issue. The portion below in red is an ADE issue not a css issue.

If you did something like

.height {
height: 100%;
height: 100vh;
}

the last height line would be the one used or ignored. Same goes for ePub3. So if you want ePub2 compatibility, vh and vw have to go. You cannot use them.

Instead, he should have said something like this:

Under ePub2
If you did something like

.height {
height: 100%;  /*accepted: height = 100%/*
height: 100vh; /*rejected - vh not supported: height remains = 100%/*
}

The last height line would be ignored because vh is not supported/understood.

Under ePub3
If you did something like

.height {
height: 100%;  /*accepted: height = 100%/*
height: 100vh; /*accepted - vh supported: height = 100vh/*
}

The last height line would be used because vh is supported and it is the most recent css following css heirarchy rules (height would be set to 100vh instead of 100%).

Unfortunately ADE (old decrepit version) is not the only device/app that doesn't follow css rules, so you must test your coding on the devices/apps that you are marketing.