If you’re reading this post, you probably already know that polyglot HTML5 is markup that can be rendered both as HTML and as XML serialized XHTML without breakage. It’s not something that’s necessary to do in EPUB 3, since the format requires the handling of content as XHTML, but if you’re planning to use your content in a general web context it’s worth knowing about.
How to make EPUB content compatible with browser rendering as HTML seems to comes up enough that I thought it would be interesting to detail. What follows is a condensed list of common issues, but for all the details the W3C maintains a polyglot markup document that should be referenced, and the WHATWG maintains similar information in its HTML vs. XHTML wiki page.
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