A common question I hear is whether EPUB3 is useless above and beyond HTML5. Why not just eliminate EPUB and use plain HTML5?
EPUB3 does bring new things to the table that HTML5 does not provide.
I see standards and technologies like ecosystems that respond to the challenges that are thrown at them. Before eBook systems, the web only had infrastructure for individual web pages that were either documents or application-like. HTML before HTML5 didn't even help much with applications, except for basic forms, so HTML5 provided features to help with this area (offline web apps, the canvas tag, etc.).
There has never been evolutionary pressure on HTML to have better long form reading for artifacts akin to eBooks, so it never really provided the facilities to help with this.
The key limitation of HTML5 is it never evolved a way to talk about anything beyond single web pages; it provides markup for individual pages, but somehow forgot to provide a way to talk about collections of pages as a unit. You can think of an eBook as simply a bundle of web pages.HTML doesn't even really have the notion of a web site baked into it, other than the security model. Google had to bake their own XML Sitemaps standard, for example, to get around this for their web crawling.
In addition, HTML in general never provided the following:
These are all the key features we associate with eBooks. EPUB3 provides solutions for each of these pieces:
None of these features come out of the box with HTML5; you either need EPUB3 for them or you have to roll your own (incompatible) personal markup above HTML5.
In the long run, I see many of the pieces of EPUB3 simply getting absorbed into a future HTML; the new name of HTML5 is The Living Standard, so it's designed exactly to absorb useful bits of functionality like this. Things like footnotes, glossaries, web pages as ZIP files, and bundles of associated web pages are all generally useful to the open web as eBooks and the web continue merging.
Unfortunately, EPUB3 has some serious problems when it comes to making it compatible with the broader HTML5 web that need to be fixed. I'll detail these in a future blog post.
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