I would say that asking about HTML5 is not within the scope of the site, as opposed for example to EPUB3: but I'd like to know what other think about it.
I think it will depend on the context of the HTML5 question, but in many cases, yes. For example, many web-based reader software, such as the Kindle Cloud Reader and the Kobo Cloud Reader are developed in HTML5.
There is a decent blog here about the relevance of HTML5 (and EPUB3) in the future of electronic reading.
Quick intro excerpt:
Digital Publishing is one of the fastest growing segments and most major publishers are now seeing 21% of their total revenues stem from it. Publishers cannot decide on what the future of publishing will entail. There are three main aspects that the vast majority of companies employing for their strategy. We are seeing strong growth with dedicated e-reading apps, HTML5, and ePub3/ePub2 as the main factors for content delivery. In the next three weeks, we will be talking to some of the leading digital publishers that are currently living in this space. They will be talking about the future and current state of digital publishing and how the industry will look in 2013-2014.
Ebook authoring is considered one of the on-topic areas of the site. By extension, so too will be HTML(5) in the context of writing ebooks. Questions purely about HTML would be off-topic here.