Apple Books provides a producing guideline for EPUB based vertical scrolling manga (Webtoon) which requires "rendition:flow" property in OPF file.
<meta property="rendition:flow">scrolled-continuous</meta>

But EPUB specs (ex: EPUB Reading Systems 3.3) says, "rendition:flow" property should not be used for fixed layout (pre-paginated).
Reading systems MUST ignore the rendition:flow property and its overrides when processing [pre-paginated spine items](https://www.w3.org/TR/epub-33/#def-layout-pre-paginated) [[epub-33](https://www.w3.org/TR/epub-rs-33/#bib-epub-33)].

Unfortunately, current EPUB spec, even 3.3, doesn't provide appropriate way to indicate Webtoon style and KADOKAWA and some Webtoon providers in Japan embeds custom meta data for Webtoon in our EPUB files. (ttb: Top to Bottom)
<meta name=“scroll-direction” content=“ttb”/>

Although I explained this situation, Amazon Kindle will also plan to require "rendition:flow" property for Webtoon in their guideline.

I don't want to give a big impact for EPUB 3.3 recommendation process and I don't want to add some new normative feature for Webtoon but I wish that we will get some reasonable solution for EPUB based Webtoon contents.

It's good for us to use common custom meta data for Webtoon like “scroll-direction” but "rendition:flow" in "pre-paginated" content will indicate Webtoon in EPUB spec officially, may be another option, I think.